Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

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Femtosecond coherent anti-Stokes Raman scattering spectroscopy of hydrogen bonded structure in water and aqueous solutions Huaning Zhu a, Yang Li a, Silvije Vdovic´ a,b, Saran Long a, Guiying He a, Qianjin Guo a,⇑ a b

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Institute of Physics, Bijenicˇka cesta 46, 10000 Zagreb, Croatia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 fsCARS spectroscopy is employed to

a r t i c l e

i n f o

Article history: Received 23 January 2015 Received in revised form 26 June 2015 Accepted 27 June 2015 Available online 29 June 2015 Keywords: Coherent anti-Stokes Raman scattering Hydrogen bonded structure Perturbation theory based numerical calculation Molecular dynamics simulations

0.6 Cl-

0.5 Intensity(a.u.)

study OH stretching (tOH) of aqueous solutions.  Numerical evaluation was applied to confirm the distinctive OH stretching mode.  Average number of hydrogen bonds per water molecule was computed.  MD simulations were used for microscopic description of the CARS spectra.  The equilibrium distributions of hydrogen bonds was analyzed in the solvation shells.

0.4 0.3 0.2 0.1 0 3000

3200

3400

3800

a b s t r a c t Femtosecond coherent anti-Stokes Raman scattering (fsCARS) spectroscopy, together with perturbation theory based numerical calculation, is employed to study OH stretching (tOH) of pure water and aqueous lithium chloride solutions. Vibrational OH stretching (tOH) modes of aqueous solutions are Raman-excited by a pair of ultrashort, femtosecond laser pulses, and then probed through inelastic scattering of a third, time-delayed laser field. In order to overcome limited spectral resolution of fsCARS, numerical evaluation of the CARS signal through vibrational wave packet propagation was employed in order to confirm the position of distinctive OH stretching mode that is complicated by intramolecular and intermolecular vibrational coupling. Moreover, in order to come to a microscopic description of the observed CARS spectra for aqueous solutions, we have performed molecular dynamics simulations of aqueous lithium chloride solutions with varying concentrations at ambient conditions. To this end we have analyzed the equilibrium distributions of hydrogen bonds in the first solvation shells of the ions as well as in bulk water and also computed the average number of hydrogen bonds per water molecule. According to our experimental and theoretical results on time evolution of Raman OH stretching band of water, it can be inferred that the dissolved ions mainly influence hydrogen bond strength and structure of water molecules in the first hydration shell, the addition of lithium chloride primarily breaks the tetrahedral hydrogen bonding, promotes formation of the donor hydrogen bonding in water, and slightly increases the amount of free OH bonds. Ó 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Guo). http://dx.doi.org/10.1016/j.saa.2015.06.115 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

3600

Wavenumber(cm-1)

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

1. Introduction With their strong intermolecular hydrogen-bonding (HB) ability, water clusters play crucial roles in many areas, such as environmental sciences, biology, chemistry, and modern technology [1–6]. Due to the complexity of the interaction of water molecules, which is dominated by the hydrogen bonding, the nature of water has not been fully understood, even though a great deal of effort has been made to investigate its distinctive properties in both experiments and theoretical studies [5–7]. In particular, the microstructure of liquid water has been and still is the subject of many experimental and theoretical studies [5–12]. Moreover, the presence of an ion in an aqueous solution may significantly influence the local water structure. It is well known that liquid water under ambient conditions has a highly structured network of hydrogen bonds. The interactions between the charged ion and the neighboring water molecules may have a distinct effect on the structure and dynamics of the water surrounding the ion [7–9]. Through changes in the equilibrium distributions of hydrogen bonds in the first and second solvation shells of the ions as well as in bulk water, the introduced ion may interfere with the structure and influence locally the behavior of water molecules, and the properties such as the ionic radius and the strength of the ion–water interaction play a central role. Since a better understanding of the properties of the solution at a microscopic level could lead to important insights into radiative damage processes or other charge induced chemical reactions, the effects of ions on hydrogen bonded network in water have been intensively investigated in the last decades [9–16]. However, it is still not clear how the structure of water is influenced by the hydrated ions and what is the size of a closed hydration shell. Resolving such ambiguities requires quantitative connections between experimental observables and the statistics of hydrogen bond geometries. Since transition frequencies are sensitive to local molecular environment, vibrational spectroscopy is particularly useful for probing OH stretching modes in water molecule [14–18]. Accordingly, various vibrational spectroscopic techniques such as Raman scattering, hole-burning, pump–probe, and coherent nonlinear vibrational spectroscopies have been employed in the last years to clarify the effects of ions on hydrogen bonded network in water [14,17–22]. However, water is an exceedingly complicated system which has more hydrogen bonds than atoms and the hydrogen bonds are fragile [23], they exist only fleetingly, and are continually broken and reformed on the picoseconds time scale [23–25]. So it should be no surprise that vibrational excitations of water exhibit complex and unusual properties. Therefore, in addition to conventional spontaneous Raman spectroscopy, various more advanced nonlinear vibrational spectroscopies can yield additional useful information about the structure of the water [26– 30]. In our investigation of LiCl aqueous solutions using the femtosecond time resolved coherent anti-Stokes Raman scattering (tr/fs CARS) technique, we found arguments that support the presence of local structures in aqueous solutions of lithium halides. Coherent anti-Stokes Raman scattering (CARS), a nonlinear four-wave-mixing process, is one of the most important third order nonlinear vibrational spectroscopic techniques, wherein the interaction of two different frequency laser pulses, xP and xS (pump and Stokes pulses) creates coherent vibrations with which a third pulse, xPr (probe pulse) on samples generates anti-Stokes signals at the frequency xAS = xP  xS + xPr that is higher than the excitation frequencies [31–33]. Typically, degenerate CARS is used, in which the pump and probe waves are the same, so that xAS = 2xP  xS. Because of the coherent vibration excited by the pump and Stokes pulses, CARS can achieve much more sensitivity than the spontaneous Raman scattering spectroscopy [32]. In

263

addition, since CARS is a third-order nonlinear process, the signal is generated mostly at the tight focus of the overlapped incident laser beams, allowing three-dimensional depth profiling and a better lateral spatial resolution [33–35]. So, it holds a great promise for obtaining detailed information about the structural changes, and there has been much effort to increase the sensitivity and selectivity of the technique [36,37]. In this paper, we report our investigation on coherent vibrational relaxation of OH stretching modes in pure water and aqueous solution of LiCl with different concentrations, measured by fsCARS at room temperature. The vibrational modes are analyzed with respect to the coordination of both donor and acceptor molecules participating in the hydrogen bond. Following a brief description of the experimental layout and the theoretical basis for the time- and frequency-resolved CARS signal described here, we address the intermolecular rearrangements that accompany solvation of ions in aqueous solution, using Raman spectroscopy of water’s O–H stretch vibrations extended with microstructural analysis based on molecular dynamics simulations. 2. Experiment A schematic of the CARS spectroscopy experimental setup is shown in Fig. 1. In our experiments, the femtosecond time resolved CARS measurements are performed at room temperature. A regenerative Ti:sapphire amplifier (Coherent Legend Elite) was seeded with a Ti:sapphire oscillator (Coherent Mira-900S) pumped by a diode-pumped, Q-switched Nd:YLF laser (Coherent Evolution, 30 W). The amplifier output consisted of 40-fs pulses centered at 800 nm with pulse energy of 2 mJ at a repetition rate of 1 kHz. The amplified output is split by a beam splitter into two beams (40% transmitted and 60% reflected). In order to obtain a wide tuning range of the Stokes beam, the reflected pulse is sent through a traveling-wave optical parametric amplifier of white light continuum (TOPAS White, Light Conversion, Vilnius, Lithuania). The transmitted pulse was used to pump a collinear optical parametric amplifier (TOPAS-C, Light Conversion, Vilnius, Lithuania), whose output was split into two beams using a 60/40 beam splitter — the pump and probe beams, respectively. The Stokes beam is 50 fs pulse tunable from 500 to 750 nm, corresponding to Raman shifts of 0–4000 cm1 for pump/probe pulses centered at 630 nm. Since pump and probe pulses have about 65 fs time duration our trCARS setup has very high time resolution while the spectral resolution in this case is limited by the probe bandwidth of about 225 cm1. To generate the CARS signal, the tr/fs CARS process requires a spatial and temporal overlap of the beams in the sample. The pump and probe pulses are delayed with respect to each other using computer controlled variable optical delay lines with a temporal resolution of 2 fs. The temporal overlap of the beams was verified using a cross-correlation setup with second harmonic generation, as well as sum frequency mixing in a thin, phase-matched b-barium borate (BBO) crystal. The correlation functions showed nearly transform limited shape of all three pulses. The position of the delay stages at which the beams coincide in time was labeled as time zero. For the trCARS, noncollinear folded-BOXCARS beam geometry (see also Fig. 1) was used to minimize the wave vector mismatch where three beams pass through the three corners of the front side of a box. The phase-matching condition is fulfilled in this geometry and after interaction with the sample, the femtosecond CARS signal emerges out from the fourth corner on the opposite box side. This ensures that the CARS signal propagates in a direction different from the three input beams and can therefore be collected avoiding inherent background from the excitation beams. In addition, time

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LN-CCD

Ti:Sa Laser 1kHZ,2mJ,40fs,800nm

Spectrum

PC anti-Stokes Signal

M1 M2

OPA1

Iris

M12

BS1

LS2

SC DS1

ks

kpu

OPA2

LS1

kas

kpr

M3

BS2 DS2

M5

M6

Stokes

Probe

M9 M10

M4 Pump M7

M8

M11

Fig. 1. Schematic optical layout for the time-resolved CARS, as described in text. DS1, 2: computer controlled delay stages; LS1, 2: focusing lenses; M1–12: alignment mirrors; LN-CCD: liquid nitrogen-cooled CCD spectrometer; BS1–3: beam splitters; SC: flow sample cell; OPA1, OPA2: optical parametric amplifier; PC: prism compressors.

delay introduced between the pump/Stokes pair of pulses and the probe pulse significantly reduces the non-resonant background due to the fact that this background signal is generated only when all three pulses overlap in time. By adjusting the probe pulse duration and its delay, non-resonant contribution is suppressed while spectral resolution is usually determined by the spectral width of the probe pulse [38,39]. The CARS signal was filtered out using a spatial filter and directed to a 300-mm focal length spectrometer (SpectraPro 2300i, Princeton Instruments) with 600 lines/mm grating. The spectrally dispersed light was detected with a liquid nitrogen-cooled CCD detector (LN/CCD-1340/400, Princeton Instruments), and the average light exposure was controlled by an electronic shutter built into the spectrometer. The CARS spectra were recorded using our custom-built data acquisition program written in LabVIEW. The sample solution was circulated in a flow cell with a 2-mm path and 1-mm thick walls to ensure that a fresh sample volume was exposed to each pump pulse. The flow rate was about 10.0 mL min1. The lithium halides used in this study are analytical grade reagents without further purification. The used water is ultra pure and has resistivity of about 18.0 MX cm. Total dissolved oxygen and organic carbon of the water are below 5.0 and 1.0 ppb, respectively. 3. Theory In the following, we briefly outline a theoretical description of our tr/fs CARS measurements. Using three ultrashort sub-100 fs pulses in our CARS experiment we obtain very high temporal

resolution that gave us high precision in measuring vibrational decoherence of the OH stretching in water molecule. On the other hand, in this case the spectral resolution is mainly limited by the bandwidth of the probe which is around 225 cm1 . It should be noted that, compared to spontaneous Raman spectra that measures time-averaged spectral response, tr/fs CARS spectra show much better instrument response product (product of time and spectral resolution of the experiment) [40]. As it will be shown later, relatively high spectral resolution is necessary for identification and analysis of time evolution of sub-bands present in the broad Raman OH stretching band. Importantly, these sub-bands are assigned to various cases of participation of the molecule to form hydrogen bonds with neighboring molecule as proton donor or proton acceptor. For that reason we performed numerical simulations of tr/fs CARS transients that will help us to identify the positions and magnitudes of overlapping sub-bands. The simulations of the tr/fs CARS transients were performed within the framework of a perturbation approach to the thirdorder polarization of the response of the system [35,37,41–46]. A schematic representation of CARS process is shown in Fig. 2. As opposed to linear diagnostic techniques such as absorption spectroscopy and LIF which involve one-photon excitations, CARS is a nonlinear diagnostic technique that relies on inducing vibrational coherence through Raman processes in the target molecule using two lasers, xP and xS (pump and Stokes pulses), which then probed by a third laser, xPr (probe pulse) generates a coherent laser-like signal in the phase-matching direction at a blue-shifted frequency. In our trCARS process, the signal is detected with a square-law detector by collecting the light that is emitted in the direction,

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

265

(c)

(a)

Kas

Ks Anti-Stokes

Probe

Stokes

Pump

Kpu

(b)

Kpr

pu

1

s

pr

2

Δ t12

3 Δ t 23

t=0

time

Fig. 2. (a) Level Scheme for CARS process. (b) Laser pulses. (c) Schematic diagram for the time-resolved fs-CARS phase-matching condition. The arrows represent the wave vectors. The third order process involves a probe pulse (xpr) delayed with respect to the Stokes/pump pulse pair (xS/xpu).These three fields generate a third order polarizability in the molecules, leading to emission of the CARS signal at the anti-Stokes frequency (xAS). All polarizations are parallel.

kAS = k1  k2 + k3. The probe delay Dt23 is scanned, while the pump delay Dt12 is held constant after optimizing the signal (See Fig. 2(b)). The detected signal is the total polarization squared,

S¼

Z

1

dTjPð3Þ ðTÞj2

ð1Þ

1

where the rapidly oscillating terms have been neglected according to the rotating wave approximation. The third-order material polarization, P(3)(t), is interrogated by monitoring the coherent anti-Stokes radiation induced with a time ordered set of three short laser pulses in the direction k1  k2 + k3. In general, four terms may contribute to the third order polarization in this direction,

D E ð3Þ ^ jwð3Þ Pk1 k2 þk3 ðtÞ ¼ wð0Þ ðt Þjl k1 k2 þk3 ðt Þ D E ^ jwkð3Þk þk ðt Þ þ wð0Þ ðtÞjl 3 2 1 D E ð2Þ ^ jwkð1Þ ðt Þ þ wk2 k3 ðtÞjl 1 D E ð2Þ ^ jwkð1Þ ðt Þ þ c:c: þ wk2 k1 ðtÞjl 3

P

D

ð0Þ

ðtÞ ¼ w

ð3Þ ðt Þj ^ jwk1 k2 þk3 ðtÞ

l

E

þ c:c:

ð2Þ

ð3Þ

τ4

kac

e' k3

g' -k2

e k1

g

ð4Þ

^ 0 denotes the atomic Hamiltonian and VðtÞ ^ denotes the where H energy of interaction of the atom with the electromagnetic field. Adopting the Born–Oppenheimer approximation, and specializ^ 0 can be written as ing to two Born–Oppenheimer states, H

^g H 0

0 ^e H

! ð5Þ

with

g

g

^ ¼H ^ 0 þ VðtÞ ^ H

^0 ¼ H

This expression includes all possible time orderings of the three interactions. The second and fourth terms are obtained from the first and third terms by interchanging the roles of pulses k1 and k3. The four terms in Eq. (2) give rise to the eight double-sided Feynman diagrams described in [42]. In our case, since the pump and probe pulse does not link to electronic resonance, the polarization for trCARS process consists mainly of one component (see Fig. 3), CARS

To treat the time-resolved nature of the CARS process numerically, a wave function description and time-dependent perturbation theory is used to derive an expression for the resonant trCARS signal. First and foremost, we consider the Hamiltonian of a molecule with two electronic states, g and e, coupled to an electric field consisting of a sequence of laser pulses. We represent the Hamiltonian for this system as:

^0 ¼ H ^g þ H ^e H X X ¼ jgnihgnjhxgn þ jemihemjhxem n

where g and e refer to the ground and excited electronic states, g and e, respectively, n and m are indices of the adiabatic rovibrational eigenstates of electronic potential surfaces g and e, respectively. When multiple fields exist for excitation, E(t) can take the general form of

EðtÞ ¼

X Ei ðt Þ ¼ 2i ðt  si Þeixi ðtsi Þþiki x

ð7Þ

i

where each pulse is centered at time si, has carrier frequency xi, is incident at wave vector ki, and has a time dependent envelope given by ei(t  si). The interaction Hamiltonian of the system in the presence of a ^Ey ðtÞ is xEx ðtÞ þ y field E ¼ ^

^ VðtÞ ¼

τ3

ð6Þ

m



0

^ x Ex ðtÞ u

^ y Ey ðt Þ u

0

 ð8Þ

with

  ^ ^ x Ex ðt Þ þ u ^ y Ey ðt Þ VðtÞ ¼ u ^ þ E ðt Þ þ u ^  Eþ ðt ÞÞ ¼ ðu

τ2 τ1

^þ ¼ u

g

Fig. 3. Double-sided Feynman diagram for P(3) in the direction k1  k2 + k3 that corresponds to CARS. Only one electronic state is considered.

X

jemihgnjlmn

ð9Þ ð10Þ

mn

^  ¼ ðu ^ þ Þy ¼ u

X jgnihemjlnm mn

ð11Þ

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

800fs

800fs

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0fs

2000

2500 3000 3500 4000 Wavenumber(cm-1)

4500

2000

2500 3000 3500 Wavenumber(cm-1)

(a)

4000

(b)

Fig. 4. Time-resolved fs CARS experimental spectra (a) and numerical calculated results (b) for various time delays between probe and actinic pump/Stoke pulses pairs of pure water. The intensities of the simulated spectra have been scaled to match those of the experimental spectra.

Table 1 The deconvolution and assignments of the Raman OH stretching vibration for water at ambient conditions. Peak No.

Assignments

Raman shifts (cm1)

1 2 3 4 5

DAA-OH DDAA-OH DA-OH DDA-OH FreeOH

3041 3250 3430 3511 3640

t

1

 E  0 ^ 0 ^ 0 Þwðn1Þ ðt 0 Þ dt eiH0 ðtt Þ=h Vðt 

D E ^ jwkð3Þk þk ðt Þ þ c:c: Pð3Þ ðtÞ ¼ wð0Þ ðtÞjl 1 2 3 Z Z t3 Z t2 D  ðÞ3 t4 dt dt dt 1 wð0Þ ðt 0 ÞflgeiHe ðtt3 Þ=h ¼ 3 2 3 1 1 ðihÞ 1  flE3 ðt3 ÞgeiHg ðt3 t2 Þ=h  flE2 ðt2 ÞgeiHe ðt2 t1 Þ=h E  flE1 ðt 1 ÞgeiHg t1 =h wð0Þ ðt 0 Þ þ c:c:

  pffiffiffi where E ðt Þ ¼ Ex ðtÞ  iEy ðtÞ = 2 are the left and right circularly ^ mn and l ^ nm are matrix elements of the dipole polarized fields. l ^ jgni. The dipole raising and lowering moment operator, lmn ¼ hemjl ^þ operators have been defined so that l mn Ei ðtÞ promotes a ket in the ground state to the excited state, corresponding to absorption of  ^ light, and l mn Ei ðtÞ demotes a ket in the excited state to the ground state, corresponding to stimulated emission. In order to find nonlinear polarizations and nonlinear susceptibilities of various orders, we use perturbation expansion in the wave function calculation, where the wave function at each order is given iteratively as

 Z  ðnÞ E w ðtÞ ¼ ðihÞ1 

Applying the perturbation theory given by Eq. (12) to third order, and using the Hamiltonian in Eqs. (4-11), we find

ð12Þ

ð13Þ

Reading Eq. (13) from right to left, we see that the initial wavepacket wð0Þ propagates on the ground surface, g, up until time t1. At time t1, the molecule interacts with the field and propagates on the excited surface e for time t2  t1; at time t2 it interacts a second time with the field and propagates on the ground surface g for time t3  t2; at time t3 it interacts a third time with the field and propagates on surface e until variable time t. The third-order wavepacket on surface e is projected onto the initial wavepacket on the ground state. This overlap is a measure of the coherence that determines both the magnitude and phase of the CARS signal. In the frequency domain, the CARS intensity, including the instrument response, is expressed as

ICARS ðxÞ ¼

Z

1

1

 2   dx1 Rðx  x1 ÞPð3Þ ðx1 Þ

ð14Þ

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

1.08ps

vDDAA

vfreeOH

1.08ps

1.04ps

1.04ps

1.00ps

1.00ps 920fs 880fs

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2000

vDDAA

267

3000 3500 4000 Wavenumber(cm-1)

4500

(a)

2500 3000 3500 Wavenumber(cm-1)

4000

(b)

Fig. 5. The fitted time-resolved fs-CARS experimental spectra (a) and calculated results (b) for various time delays between probe and actinic pump/Stoke pulses pairs of 1 M LiCl solution. The intensities of the simulated spectra have been scaled to match those of the experimental spectra.

where P(3)(x) represents the Fourier transformation of the time domain polarization P(3)(t). R(x) denotes the Gaussian spectrometer response which can be expressed as

RðxÞ ¼ exp 4 ln 2

x2

!

D2irf

ð15Þ

Here, Dirf is the FWHM of the instrument response function. 4. Results and discussions There are many physical properties such as the viscosity or conductivity of pure water that change significantly when salts are dissolved [47]. Naturally these changes depend on ionic charge and size of the dissolved salt, but what is of greatest interest is to see how changes of macroscopic physical properties are reflected in the microscopic structure and how the structure of water is influenced by the hydrated ions. Early studies indicate that dissolved ions significantly affect hydrogen bond (HB) strength and geometry even for molecules beyond the first solvation shell [48]. However, recent computer simulations and experimental measurements suggested that dissolved ions mainly influence orientational and vibrational relaxation times of water molecules in the first hydration shell [9,49], implying that structural rearrangements are strongly localized. To resolve such ambiguities, quantitative connections between experimental observables and the statistics of HB geometries are required. In this contribution, we address the intermolecular rearrangements that accompany solvation of lithium and chloride anions in aqueous solution, using Coherent anti-Stokes Raman scattering

spectroscopy of water’s O–H stretch vibrations together with molecular dynamics (MD) simulations. The frequency of the OH stretching mode is very sensitive to its molecular surrounding [50]. Changes induced in the system like pressure, hydrated salts or temperature gradients can change the intensity and position of the band within the spectrum significantly [50]. Accordingly, this work applied tr/fs CARS experimental technique to probe OH stretching vibrations (tOH) of water and lithium chloride solutions with different concentrations. The experimental spectrally dispersed tr/fs CARS signal as a function of the probe delay for pure water is shown in Fig. 4(a). As we have discussed above, the band position of the OH stretching vibration (tOH) of liquid water can be assigned to a useful quantitative marker of the magnitude of hydrogen bonds. However, from Fig. 4(a), we can see that vibrational line shapes for the OH stretch vibration of water are complicated by intramolecular and intermolecular vibration coupling besides the effects of motional narrowing. The effects of any intramolecular and intermolecular vibrational coupling might be expected to be substantial in respect that all OH stretch transition frequencies in the liquid water are degenerate in a zeroth-order local-mode picture. From the previous theoretical studies and detailed analyses [51–64], the local hydrogen-bonded network can be differentiated by the participation of the molecule to form hydrogen bonds with the neighboring molecule either as proton donor (D), proton acceptor (A), or their combinations. Accordingly, for water under ambient experimental conditions, five main sub-bands located at 3041, 3250, 3430, 3511, and 3640 cm1 in Fig. 4 can be discriminated as DAA (single donor–double acceptor), DDAA (double donor–double acceptor), DA (single donor–single acceptor), DDA

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1.12ps

vDAA-OH vDA-OH

1.04ps

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1.12ps

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40fs 0fs

2000

0fs

2500 3000 3500 4000 Wavenumber(cm-1)

4500

(a)

2500

3000 3500 Wavenumber(cm-1)

4000

(b)

Fig. 6. The fitted time-resolved fs-CARS experimental spectra (a) and calculated results (b) for various time delays between probe and actinic pump/Stoke pulses pairs of LiCl solution with 4 M. The intensities of the simulated spectra have been scaled to match those of the experimental spectra.

(double donor–single acceptor) and free OH symmetric stretching vibrations (See Table 1), respectively. This implies that besides three-dimensional hydrogen bonding, there should also exist chain or ring hydrogen bonding structure in liquid water. Based on resolving O–H stretch vibrations components in Coherent anti-Stokes Raman scattering spectra of pure water, we studied the series of lithium chloride solutions with different concentrations, to see whether the presence of an ion in an aqueous solution may significantly influence the local water structure. The experimental spectrally dispersed tr/fs CARS signals as a function of the probe delay for lithium chloride solutions with concentration in the range 1–11 M are shown in Figs. 5(a), 6(a) and 7(a), respectively. The corresponding numerical calculated results are shown in Figs. 5(b), 6(b) and 7(b), respectively. The major peaks and coherence period seen in the experimental results, Figs. 5(a), 6(a) and 7(a), can be well reproduced by the calculations in Figs. 5(b), 6(b) and 7(b), respectively. Combined experimental analysis and numerical evaluation, shows that the increase of concentration causes the decrease of intensity of the 3250 cm1 band at a constant wavenumber, and slightly lowers the intensity of higher wavenumber (>3625 cm1) sub-bands. The 3430 and 3511 cml bands intensity are increased compared to the 3250 cml band, and their wavenumber changes to a higher value. In order to see the influence of ionic concentration on the tetrahedral hydrogen bonded structure of water, we compared the coordination number around water for LiCl concentrations varying from 1 M to 11 M with that of pure water, which can be determined from the local statistical model of liquid water. For this purpose, the combination of Coherent anti-Stokes Raman scattering experiments and quantum-chemical simulation of liquid water

were used to assess the intensity of different vibrational modes in the valence band of OH groups for each of the five sub-band components, and the intensities of vibrational modes for the water cluster were normalized to their concentration in liquid water. The next step was to determine the proportional contribution of the normalized intensity of each vibrational mode for a certain cluster types in a separate component. Therefore, the mean hydrogen bonding number can be determined as

NHB ¼ 4  IDDAA þ 3  IDDA þ 3  IDAA þ 2  IDA

ð16Þ

where IDDAA ; IDDA ; IDAA ; IDA are the percent contribution of the each separate component for lithium chloride solutions with different concentrations. From numerical analysis of experimental results for Raman OH stretching band of lithium chloride solutions with different concentrations at ambient conditions, the hydrogen bonding concentrations can be determined, and then the mean hydrogen bonding number can be calculated using Eq. (16) as represented in Table 2. In the calculations, we used known vibrational frequencies and vibrational decoherence lifetimes consistent with the observed decay constants from the tr-CARS data, as the linewidths in CARS spectra depend on a convolution of the bandwidth of the probe pulse and the natural linewidths associated with vibrational decoherences. The pulse envelopes are taken to be of Gaussian form with the intensity full width at half maximum (FWHM) set to 50 or 100 fs. The time interval Dt12 was set to 50 fs, the phase differences between the pulses were zero. The probing time Dt23 is varied between 0 and 15.0 ps, i.e., the third-order polarization is computed for each Dt23 (in 10 fs steps) and saved. From Table 2, we can see that the percent contribution of the each type

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

vDDA-OH

1.04ps

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920fs

880fs

880fs

840fs

840fs

800fs

800fs

760fs

760fs

720fs

720fs

680fs

680fs

640fs

640fs

600fs 560fs 520fs 480fs

600fs 560fs 520fs 480fs

440fs

440fs

400fs

400fs

360fs

360fs

320fs

320fs

280fs

280fs

240fs

240fs

200fs

200fs

160fs

160fs

120fs

120fs

80fs

80fs

40fs

40fs

0fs

0fs

2000

2500

3000

3500

vDDA-OH

1.16ps

Intensity(a.u)

Intensity(a.u)

1.16ps

4000

269

2500

4500

Wavenumber(cm-1)

3000

3500

4000

Wavenumber(cm-1)

(a)

(b)

Fig. 7. The fitted time-resolved fs-CARS experimental spectra (a) and calculated results (b) for various time delays between probe and actinic pump/Stoke pulses pairs of LiCl solution with 11 M. The intensities of the simulated spectra have been scaled to match those of the experimental spectra.

3.5 pure water 1.06M 3.97M 7.94M 10.85M

3

gOO(r)

2.5 2 1.5 1 0.5 0

1

2

3

4

r(Å)

5

6

7

8

9

Fig. 8. The oxygen–oxygen radial distribution functions for LiCl solutions at ambient conditions for four different concentrations compared with pure water.

of the water cluster in a separate component change significantly when LiCl dissolves in water. With the increase of concentration, the percent contribution of DDAA and DAA hydrogen bonding are highly decreased, while the percent contribution of DA and DDA

hydrogen bonding are increased clearly, and the percent contribution of free OH slightly lowers. We also can see that the increase of concentration causes the decrease of the mean hydrogen bonding number from 2.94 to 2.47 at ambient condition.

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1.8 pure water 1.06M 3.97M 7.94M 10.85M

1.6 1.4

gOH(r)

1.2 1 0.8 0.6 0.4 0.2 0

1

2

3

4

r(Å)

5

6

7

8

9

Fig. 9. The hydrogen–oxygen radial distribution functions for LiCl solutions at ambient conditions for four different concentrations from 1 M to 11 M compared with pure water.

Table 2 Fitting results of the CARS spectrum of pure water and lithium chloride solutions at various concentrations in the OH stretching region under ambient conditions. Peak No.

Assignments

Raman Shifts (cm1)

1 2 3 4 5

DAA DDAA DA DDA FreeOH

3041 3250 3430 3511 3640

Hydrogen bonding number(NHB)

In order to come to a microscopic description of the observed tr/fs CARS spectra for lithium chloride solutions with different concentrations, we have carried out MD simulations of aqueous lithium chloride solutions to investigate the changes of the hydrogen bonded structures in the vicinity of ions for different ion concentrations. In MD simulations, the property that we investigate is the distributions of water molecules in the whole simulation box with a given number of donor or acceptor HBs. The water molecules were represented by the flexible extended simple point charge (SPC/E) model [65]. The charges and Lennard-Jones parameters according to models developed by Dang [66] were used to reproduce the experimental density, energy, and oxygen–oxygen radial distribution function of water. The OPLS (Optimized Potential for Liquid Simulations) parameters were used for Cl– water interactions [67]. During the entire simulation run, the Nosé–Hoover thermostat was adopted for temperature control, and the Parrinello–Rahman algorithm was performed to maintain the constant pressure [68–70]. The simulation lengths were 500 ps for the calculation of radial distribution functions and the simulation time step was 1 fs. A cubic simulation cell was constructed containing 210 water molecules of AB4 model (42 (H2O)A and 168 (H2O)B) were used. As we discussed above, liquid water under ambient conditions is known to have a highly structured network of hydrogen bonds. The presence of an ion in an aqueous solution will interfere with this structure and may influence locally the behavior of water molecules. For liquid water, a water molecule should interact with

Hydrogen bonding concentrations Pure water (%)

1(M) (%)

4(M) (%)

11(M) (%)

12.9 44.5 17.1 14.4 11.2

12.4 41.1 20.7 14.6 11.2

11.1 33.6 29.3 15.2 10.8

9.8 20.2 41.4 18.1 10.5

2.94 ± 0.15

2.87 ± 0.15

2.72 ± 0.15

2.47 ± 0.15

neighboring water molecules through various local hydrogen bondings. As LiCl dissolves in water, water molecules form hydration shells around the Li+ and Cl ions. Therefore, the dissolved Li+ and Cl undoubtedly affect the water structure, and cause spectral changes in CARS signal. Naturally, effects of these ions on water structure should be different. The XAS studies by Cappa et al. [61,71] proposed that XAS spectral changes are caused by direct electronic perturbation by the anions on the surrounding water molecules while monovalent cations have no significant effect on the structure of water. Their work indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water–chloride interactions are primarily responsible for the observed spectral changes [67,68]. Based on their work, only the effect of Cl on the water structure has been investigated whereas effect of Li+ was neglected in this work. The oxygen–oxygen, oxygen–hydrogen, chloride ion–oxygen and chloride ion–hydrogen radial distribution functions (RDFs) for LiCl concentrations varying from 1 M to 11 M were compared with that of pure water to see the influence of ionic concentration on the structure of water cluster. It can be seen from Fig. 8 that, three peaks for all concentrations is corresponded to first, second and third nearest neighbors in the radial distribution function g oo ðrÞ, and the change in concentration have negligible effect on the intermolecular distance of oxygen atoms represented by the ROO peaks around 2.8 Å and 4.5 Å. The oxygen and hydrogen RDFs for 1–11 M LiCl solutions showed in Fig. 9 informs about

H. Zhu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 262–273

271

3.5 1.06M 3.97M 7.94M 10.85M

3

gClO(r)

2.5 2 1.5 1 0.5 0

1

2

3

4

r(Å)

5

6

7

8

9

Fig. 10. The chloride–oxygen radial distribution functions for four LiCl concentrations from 1 M to 11 M at ambient conditions.

3 1.06M 3.97M 7.94M 10.85M

2.5

gClH(r)

2

1.5

1

0.5

0

1

2

3

4

r(Å)

5

6

7

8

9

Fig. 11. The chloride–hydrogen radial distribution functions for four LiCl concentrations from 1 M to 11 M at ambient conditions.

Table 3 Average coordination numbers of oxygen atoms and hydrogen bonds of water in first solvation shell of aqueous LiCl solution at various concentrations. Concentration

hnOOi

hnClHi

hn cli o

hnHBi

Pure water 1.06 M 3.97 M 7.94 M 10.85 M

4.44 4.86 5.37 5.61 5.72

– 7.26 7.10 6.95 6.89

– 7.48 7.60 7.76 7.81

3.20 2.94 2.78 2.67 2.48

hydrogen bonds number. The obtained intermolecular length value of this hydrogen bond at around 1.9 Å for first peak and 3.2 Å for the second do not change with variation of the concentration. In order to look into the changes in the RDFs associated with the chloride anion for different concentration, the chloride ion–oxygen and

chloride ion–hydrogen RDFs for the 1–11 M LiCl solutions are shown separately in Figs. 10 and 11. It is seen from Figs. 10 and 11 that, for chloride ion–oxygen and chloride ion–hydrogen pair correlations, although the peak heights change to some extent with increase of chloride ion concentration, the locations of the first and second minima remain essentially unchanged and hence the shell radii were taken to be of the same value for all concentrations. Here, the average coordination number is calculated by the integral that mainly focused on the first solvation shell:

n¼

Z

Dmin

4pr 2 qg ðrÞdr

ð17Þ

0

where q is the molecular density, and Dmin is the first minimum distance of radial distribution function gðrÞ.

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The obtained results for average coordination numbers of the hydrogen bonds and oxygen atoms of water in the first coordination shell of the aqueous solution LiCl with various concentrations at ambient conditions are reported in Table 3. It is seen from Table 3 that, with the increase in concentration from 1 M up to 11 M, the average coordination numbers of oxygen atoms hnOOi number increases from 4.86 up to 5.72, which reveal that the tetrahedral structure of water model is broken by the presence of ions. In Table 3, we also show calculated average coordination numbers of oxygen and hydrogen atom around Cl for each molarity in the first solvation shell. From Table 3, we can see that the average coordination numbers of oxygen atoms around Cl in the first coordination shell slightly increases from 7.48 to 7.81, while the average coordination number of hydrogen around Cl decreases slightly from 7.26 to 6.89 as the lithium chloride concentration is increased. Table 3 also shows us that increase of lithium chloride concentration is accompanied by a reduction of average numbers of hnHBi hydrogen bonds. For pure water the hnHBi number is 3.20 and decreases to 2.48 for larger molarity. This is almost consisting with above CARS experiment which gives the mean hydrogen bonding number from 2.94 down to 2.47 with the increase in concentration (See Table 2). So, we can conclude that the dissolved ions mainly influence hydrogen bond strength and structure of water molecules in the first hydration shell. In this contribution, we mainly study the intermolecular rearrangements that accompany solvation of chlorine anion with different concentration in aqueous solution, using Raman spectroscopy of water’s O–H stretch vibrations together with computer simulations. The MD simulations and experimental study of lithium chloride solutions revealed that with increase in ionic concentration, chloride ion–water hydrogen bonds replace water–water hydrogen bonds to some extent accompanied with the changes in the average number of hydrogen bonds per water molecule in the first hydration shell. It can be inferred that the chloride ion– water hydrogen bonds are formed in place of some of the water– water hydrogen bonds with increase in ionic concentration, which can further explain that the addition of Cl primarily breaks the tetrahedral hydrogen bonding and promotes formation of the donor hydrogen bonding in water, and slightly lowers the amount of free OH bonds.

ions mainly influence hydrogen bond strength and structure of water molecules in the first hydration shell. Acknowledgments This work was supported by the 973 Program (2013CB834604), NSFCs (21173235, 21127003, 21333012, and 21373232). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

5. Conclusions In this study, Coherent anti-Stokes Raman scattering (CARS) spectroscopy, together with the numerical simulation of CARS process using a wave function description and time-dependent perturbation theory, is employed to study the effects of dissolved lithium chloride on water structure. The aim of this study has been to address the issue of how ions alter the hydrogen bonded network of water at different concentrations. According to our interpretation of Raman OH stretching band of water, it can be inferred that the addition of lithium chloride primarily breaks the tetrahedral hydrogen bonding and promotes formation of the donor hydrogen bonding in water, and slightly lowers the amount of free OH bonds. This is primarily due to replacement of water–water hydrogen bonds, with water–chloride hydrogen bonds. A very good agreement was obtained between the calculated and experimental Raman spectra of water and lithium chloride solvent. Moreover, we have presented molecular dynamics simulations of aqueous lithium chloride solutions of varying concentrations under ambient conditions. To this end we have analyzed the equilibrium distributions of hydrogen bonds in the first solvation shells of the ions as well as in bulk water and also computed the average number of hydrogen bonds per water molecule. We conclude that dissolved

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