Article pubs.acs.org/JPCA
Complexation of Allyl Radicals and Hydrochloric Acid in Helium Nanodroplets Daniel Leicht, Daniel Habig, Gerhard Schwaab, and Martina Havenith* Department of Physical Chemistry II, Ruhr-University Bochum, 44801 Bochum, Germany ABSTRACT: Infrared spectra of the allyl radical−HCl complex in superfluid helium nanodroplets have been recorded in the IR frequency range of 2750−3120 cm−1. Six fundamental bands were observed, five of which have been assigned to the C−H stretch vibrations of the allyl radical. No additional CH bands were observed upon the binding of HCl. The band at 2800.3 cm−1 can unambiguously be assigned to the bound HCl stretch, which is red-shifted by 106 cm−1 compared to that of the free HCl. Stark spectra and pickup curves were recorded and support our assignment. In accompanying ab initio calculations, we found four equivalent minima and computed a two-dimensional potential energy surface for the HCl positioning on the allyl radical plane at the CCSD(T)/TZVPP level. Based on our findings, we conclude that the ground-state structure of the complex shows two energetically equivalent T-shaped minimum structures. Because of small barriers between the two minima, a delocalization of the HCl is anticipated.
1. INTRODUCTION The allyl radical is one of the simplest hydrocarbon radicals that shows π-conjugation.1 Previous studies include both computational2,3 and experimental techniques, including argon matrix isolation,4,5 jet expansion,6,7 and recently helium nanodroplet spectroscopy.8 Allyl radicals play a key role in reactions of explosives and combustion of organic compounds. Additionally, they are important in tropospheric chemistry.9 Their reactions, especially with molecular oxygen, were investigated in a series of studies.8−10 Because of the importance of the allyl radical in many chemical processes, it is necessary to improve the understanding of its interactions with other species. Presumably, the delocalized electronic system will serve as a binding site, similar to the π-system in benzene.11 To study the ability of the allyl radical to function as a hydrogen-bond acceptor, we chose hydrochloric acid as a simple model system. In a previous study of HF−CH3, Miller and co-workers characterized the exit channel of the hydrogen abstraction by fluorine.12 In the case of HF−CH3, hydrogen bonding resulted in a red shift of the HF stretch of 162 cm−1. In comparison, because of the vast electron density along the carbon chain of the allyl radical, we expect a higher binding energy for the allyl−HCl complex. The technique of helium nanodroplet spectroscopy is a wellestablished experimental technique for studying weakly bound complexes as well as radicals.13−16 Because the superfluid helium hardly interacts with the embedded molecules, solute molecules are able to rotate and vibrate almost freely. This results in a very small matrix-induced shift17 and allows a direct comparison to ab initio predictions.18 It is well-known that a partial fraction of the helium is not in its superfluid state and induces a friction-like effect, which reduces the effective rotational constants of the molecules.15 © 2015 American Chemical Society
In this article, we present IR spectroscopic measurements of the allyl−hydrochloric acid (1:1) aggregate. The wavelength range examined in this work (2750−3120 cm−1) covers the region of both the C−H and HCl stretching vibrations. Six fundamentals are expected to be located within this wavelength range: five allyl C−H vibrations and one HCl stretch vibration. The experimental results are complemented by ab initio calculations to gain a better understanding of the structural and dynamical properties of the complex. Conformational searches from random structures, geometry optimizations, potential energy surface scans, frequency calculations, and experimental data provide a complete picture of the allyl−HCl complex.
2. METHODS 2.1. Helium Droplet Apparatus. Measurements were carried out using our home-built helium nanodroplet spectrometer combined with a commercially available continuous-wave infrared optical parametric oscillator (OPO) (Lockheed-Martin Aculight ARGOS 2400) providing radiation in the 3.2−3.9-μm range. The spectrometer setup is described in detail elsewhere.16,19 Only a brief overview is given here: In the expansion chamber, superfluid helium nanodroplets are formed in a supersonic expansion of ultrapure, precooled helium through a 5-μm nozzle. The skimmed helium nanodroplet beam passes four differentially evacuated chambers: two pickup chambers that allow us different species to be embedded sequentially, a multipass chamber where Stark spectra can be recorded using perpendicular laser excitation combined with a strong dc electric field, and a mass spectrometer chamber where a Pfeiffer QMS 422 quadrupole Received: November 23, 2014 Revised: January 16, 2015 Published: January 19, 2015 1007
DOI: 10.1021/jp511708s J. Phys. Chem. A 2015, 119, 1007−1012
Article
The Journal of Physical Chemistry A mass spectrometer is located for detection. When the embedded molecules are in resonance with the laser radiation, the absorbed energy will be transferred to the helium droplet. This leads to the evaporation of helium atoms from the droplet and a depletion of the mass spectrometer signal due to a decrease in droplet cross section for electron-beam ionization. The mass spectrometer can be used either in high-pass mode, detecting all masses with m/z ≥ 8, or in band-pass mode, detecting only one specific mass. For the measurements described here, the helium gas was cooled to 18 K at a stagnation pressure of 60 bar. For these expansion conditions, average droplet sizes of about 5000 He atoms can be estimated.20 The first pickup chamber contained the allyl radicals, whereas the second was filled with gaseous hydrochloric acid. The IR laser beam was amplitude-modulated by a chopper at a frequency of 31 Hz, allowing for phasesensitive detection of the depletion signal by a lock-in amplifier. 2.2. Radical Generation. Allyl radicals were generated with 1,5-hexadiene as the precursor in the first pickup chamber using an effusive thermal source for pyrolysis that consisted of a quartz tube of 15-cm length. The pyrolysis oven could be heated to 1300 K; the temperature was kept constant with an accuracy of
Complexation of Allyl Radicals and Hydrochloric Acid in Helium Nanodroplets Daniel Leicht, Daniel Habig, Gerhard Schwaab, and Martina Havenith* Department of Physical Chemistry II, Ruhr-University Bochum, 44801 Bochum, Germany ABSTRACT: Infrared spectra of the allyl radical−HCl complex in superfluid helium nanodroplets have been recorded in the IR frequency range of 2750−3120 cm−1. Six fundamental bands were observed, five of which have been assigned to the C−H stretch vibrations of the allyl radical. No additional CH bands were observed upon the binding of HCl. The band at 2800.3 cm−1 can unambiguously be assigned to the bound HCl stretch, which is red-shifted by 106 cm−1 compared to that of the free HCl. Stark spectra and pickup curves were recorded and support our assignment. In accompanying ab initio calculations, we found four equivalent minima and computed a two-dimensional potential energy surface for the HCl positioning on the allyl radical plane at the CCSD(T)/TZVPP level. Based on our findings, we conclude that the ground-state structure of the complex shows two energetically equivalent T-shaped minimum structures. Because of small barriers between the two minima, a delocalization of the HCl is anticipated.
1. INTRODUCTION The allyl radical is one of the simplest hydrocarbon radicals that shows π-conjugation.1 Previous studies include both computational2,3 and experimental techniques, including argon matrix isolation,4,5 jet expansion,6,7 and recently helium nanodroplet spectroscopy.8 Allyl radicals play a key role in reactions of explosives and combustion of organic compounds. Additionally, they are important in tropospheric chemistry.9 Their reactions, especially with molecular oxygen, were investigated in a series of studies.8−10 Because of the importance of the allyl radical in many chemical processes, it is necessary to improve the understanding of its interactions with other species. Presumably, the delocalized electronic system will serve as a binding site, similar to the π-system in benzene.11 To study the ability of the allyl radical to function as a hydrogen-bond acceptor, we chose hydrochloric acid as a simple model system. In a previous study of HF−CH3, Miller and co-workers characterized the exit channel of the hydrogen abstraction by fluorine.12 In the case of HF−CH3, hydrogen bonding resulted in a red shift of the HF stretch of 162 cm−1. In comparison, because of the vast electron density along the carbon chain of the allyl radical, we expect a higher binding energy for the allyl−HCl complex. The technique of helium nanodroplet spectroscopy is a wellestablished experimental technique for studying weakly bound complexes as well as radicals.13−16 Because the superfluid helium hardly interacts with the embedded molecules, solute molecules are able to rotate and vibrate almost freely. This results in a very small matrix-induced shift17 and allows a direct comparison to ab initio predictions.18 It is well-known that a partial fraction of the helium is not in its superfluid state and induces a friction-like effect, which reduces the effective rotational constants of the molecules.15 © 2015 American Chemical Society
In this article, we present IR spectroscopic measurements of the allyl−hydrochloric acid (1:1) aggregate. The wavelength range examined in this work (2750−3120 cm−1) covers the region of both the C−H and HCl stretching vibrations. Six fundamentals are expected to be located within this wavelength range: five allyl C−H vibrations and one HCl stretch vibration. The experimental results are complemented by ab initio calculations to gain a better understanding of the structural and dynamical properties of the complex. Conformational searches from random structures, geometry optimizations, potential energy surface scans, frequency calculations, and experimental data provide a complete picture of the allyl−HCl complex.
2. METHODS 2.1. Helium Droplet Apparatus. Measurements were carried out using our home-built helium nanodroplet spectrometer combined with a commercially available continuous-wave infrared optical parametric oscillator (OPO) (Lockheed-Martin Aculight ARGOS 2400) providing radiation in the 3.2−3.9-μm range. The spectrometer setup is described in detail elsewhere.16,19 Only a brief overview is given here: In the expansion chamber, superfluid helium nanodroplets are formed in a supersonic expansion of ultrapure, precooled helium through a 5-μm nozzle. The skimmed helium nanodroplet beam passes four differentially evacuated chambers: two pickup chambers that allow us different species to be embedded sequentially, a multipass chamber where Stark spectra can be recorded using perpendicular laser excitation combined with a strong dc electric field, and a mass spectrometer chamber where a Pfeiffer QMS 422 quadrupole Received: November 23, 2014 Revised: January 16, 2015 Published: January 19, 2015 1007
DOI: 10.1021/jp511708s J. Phys. Chem. A 2015, 119, 1007−1012
Article
The Journal of Physical Chemistry A mass spectrometer is located for detection. When the embedded molecules are in resonance with the laser radiation, the absorbed energy will be transferred to the helium droplet. This leads to the evaporation of helium atoms from the droplet and a depletion of the mass spectrometer signal due to a decrease in droplet cross section for electron-beam ionization. The mass spectrometer can be used either in high-pass mode, detecting all masses with m/z ≥ 8, or in band-pass mode, detecting only one specific mass. For the measurements described here, the helium gas was cooled to 18 K at a stagnation pressure of 60 bar. For these expansion conditions, average droplet sizes of about 5000 He atoms can be estimated.20 The first pickup chamber contained the allyl radicals, whereas the second was filled with gaseous hydrochloric acid. The IR laser beam was amplitude-modulated by a chopper at a frequency of 31 Hz, allowing for phasesensitive detection of the depletion signal by a lock-in amplifier. 2.2. Radical Generation. Allyl radicals were generated with 1,5-hexadiene as the precursor in the first pickup chamber using an effusive thermal source for pyrolysis that consisted of a quartz tube of 15-cm length. The pyrolysis oven could be heated to 1300 K; the temperature was kept constant with an accuracy of
