Article pubs.acs.org/JPCA
Infrared Photoisomerization of 1‑Propanol CD3 and OD Trapped in Four Cryogenic Matrices: Ne, N2, Ar, and Xe J. A. Noble and S. Coussan* Laboratoire Physique des Interactions Ioniques et Moléculaires, UMR 7345-CNRS, Aix-Marseille Université, Centre St-Jérôme, 13397 Marseille Cedex 20, France ABSTRACT: The conformational equilibria and isomerization processes of 1propanol -OD and -CD3 have been studied by vibrational spectroscopy at low temperatures in four cryogenic matrices to investigate the effect of deuteration on their photochemistry. These isotopic species were selectively irradiated in the νOH and νOD domains, resulting in the identification of several conformers that are able to interconvert upon selective IR irradiation. The experimental results were compared with theoretical geometries obtained at the B3LYP/6-311+G(d) level of theory. Alkyl chain isomerization can be induced in rare gas and nitrogen cryogenic matrices by suitable selective irradiation. Selective excitation of the OH and OD stretches of two Gauche isomers transfers the alkyl chain from the gauche to the trans form. The competition between intramolecular vibrational energy relaxation and the matrix-dopant interaction determines the torsional subspace dynamics of the vibrationally excited propanol molecules.
A. INTRODUCTION 1-Propanol, hereafter referred to as 1-PrOH, is a really interesting species with which to probe the subtle conformational isomerizations of alkyl chains and OH torsions.1 Indeed, as presented in a previous publication,1 in a molecular jet we were able to observe only alkyl chain isomerization, while in cryogenic matrices, upon selective IR νOH irradiation, we observed both alkyl chain and OH torsion isomerizations. Moreover, in cryogenic matrices, we showed that 1-PrOH photochemistry is controlled by the nature of the matrix. However, considering that one of the main results observed in cryogenic matrices is the isomerization of the Gt isomer (Figure 1), the most stable isomer whatever the level of calculation,2−4 toward Tg and Tt forms (in rare-gas and nitrogen matrices) or Gg forms (in CCl4), it is
interesting to probe the effect of deuteration on 1-PrOH photoreactivity. Indeed, if we consider the effects recorded in rare gas matrices or in nitrogen, the unique 1-PrOH IR photoreactivity observed after deposition is Gt → Tt,Tg. This effect is triggered by the internal H-bond between one free oxygen doublet and the methyl group. This H-bond mediates the vibrational energy transfer toward the methyl, inducing the alkyl chain isomerization. Although the effect of methyl deuteration has been addressed in the case of the dielectric relaxation of 1-PrOH,5 this effect has been more extensively studied in the case of deuteration6 or fluorination7 of a methyl rotor and its consequences on internal vibrational relaxation (IVR) and vibrational energy transfer (VET) times. In these studies it is observed that unexpectedly both deuteration and fluorination of the methyl rotor accelerate the IVR and that IVR is much faster than VET. More pertinent to our current study is the investigation of the deuteration of methyl and OH acetic acid groups carried out by Maçôas et al.8 In their study they induce, in Ar, Kr, and Xe cryogenic matrices, photoisomerization between the trans and the cis isomers by selective irradiation of the 2νOH mode of the trans; the photoproduced cis isomer tunnels back to the trans one. However, they note that the quantum yields of the trans → cis and cis → trans reactions are not affected by either the mode selectively irradiated or the matricial gas. When they carry out experiments on the d3-methyl or the OD species, they observe that methyl deuteration increases the trans → cis quantum yield, while OH deuteration decreases it. They explain these results in terms of the changes in coupling rates between Received: December 18, 2014 Revised: January 23, 2015
Figure 1. Structures and relative energies of the propanol conformers calculated at B3LYP/6-311+G(d) level of theory. © XXXX American Chemical Society
A
DOI: 10.1021/jp5126359 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Table 1. Experimental Vibrational Wavenumbers ν̃/cm−1 of 1-PrOD Gt in the Four Cryogenic Matrices Compared with Theoretical Predictions B3LYP/6-311+G(d)a B3LYP/6-311+G(d) characterb
ν̃
Ne
Ar
Xe
N2
δOD/δCH δOD
815.1 (9.0) 942.9 (17.8)
930.3
982.2 (63.9)
970.6
926.7 929.0 967.5c 971.0c 1077.5c
935.9
νCO
928.1 929.6 968.4c 969.1c 1079.7c
970.6 971.7 1081.6
2699.5c 2704.3
2687.6c
2698.6
νCC/CO
1088.4 (24.2)
νOD
2761.4 (10.5)
Δν̃
106.2
1080.8 1082.2 2716.8
Theoretical IR intensities (km mol−1) are given in parentheses. Δν̃/cm−1 are wavenumber differences between the underlined bands that support the assignment from difference spectra. bApproximative description. cTentative assignment. a
propanol in solution at room temperature. They also observe that only a small percentage of the OH excitation is transferred to the CH stretch mode. In light of the studies previously highlighted, we present an indepth study of the mechanisms involved in the vibrational relaxation of 1-PrOH. Indeed, as previously written, either the internal H-bond or the nature of the matrix plays a key role in the photochemistry of 1-PrOH. Contrary to what is seen in solution,10 the fact that we observe alkyl chain isomerization only when there is an internal H-bond seems to indicate a first locked vibrational relaxation step leading to the isomerization through this medium. Although a “through-bond transfer” is involved, it does not lead to alkyl chain isomerization when the methyl CH stretch energy is much higher than the highest isomerization energy.1 The key role of the H-bond is also illustrated by the fact that we never observe a back isomerization toward Gt. Additionally, why are we able to observe further Tt ⇄ Tg isomerization in the nitrogen matrix only? The thermal cryogenic bath is evidently crucial to 1-PrOH photochemistry. One of the most relevant methods is to study deuterated species to address these questions and more deeply understand the vibrational relaxation of 1-PrOH in solid cryogenic media. We report the results of a study of d3-methyl and OD 1-PrOH trapped in four cryogenic matrices. Carrying out selective IR irradiations in the νOH and νOD regions, we are able to provide isomer identification, compare the results with those of 1-PrOH,1 and bring new insight into the global understanding of the vibrational relaxation pathways of the deuterated species.
vibrational levels, which induce an increase in the vibrationally excited-state lifetime in d3-methyl acetic acid and therefore in the isomerization probability. Another study of great interest is that of Yamada et al.9 In their highly detailed study on IVR, vibrational energy relaxation (VER), vibrational predissociation (VP), and intramolecular vibrational energy transfer (IVET) in isolated molecules and clusters, they studied VET in phenylalcanols, C6H5(CH2)n, n = 1−6. The first problem they addressed was determining whether exciting the OH stretching mode, CH2, is an effective method to transfer OH vibrational excitation toward the ring. They showed that the C6H5CD2OH compound presents an IVR 1.7 times larger than that of the nondeuterated molecule, validating the assertion that CH2 is indeed a conduit for “through-bond” vibrational energy transfer. The second issue they addressed, by studying chain length (n) dependence, is “whether the IVET occurs step by step in each CH2 group”. The systematic addition of CH2 groups does not lead to a systematic decrease in the IVR; indeed, the shortest IVET time is found for 4-phenyl butanol. However, except for the deuterated species, the IVET times of all of the species (n= 1−6) are not so different. The authors explain this as a result of the strong coupling between the CH2 stretching vibration and the wagging in which all of the CH2 groups move in phase. As a result, after OH excitation, IVET “occurs immediately within the alkyl chain and the rate is independent of the chain length”. The fact that the shortest time is found for the 4-phenyl butanol is explained as a conformer dependence of the IVET. The study of Wang et al.10 on vibrational energy transfer in different alcohols (including 1-propanol) in solution is of particular interest with respect to our current study. They explored the IVR of ethanol, 1- and 2-propanol, and 1-butanol between the initially excited OH and the methyl terminal group. They observe that each CH2 group addition lengthens IVR by 0.4 ps. They also suggest that “through-space VET”, between the OH and the CH stretching transitions, is really insignificant compared with the nonradiative effects lifetimes (
