Antero Aspiala

IR-induced photorotamerization of 2-aminoethanol in low-temperature matrices. AB initio optimized geometries of conformers

Abstract The νOH and πOH regions of 2-aminoethanol have been studied in low-temperature matrices. In the vapour phase and in Ar, Kr, Xe and N2 matrices this compound exists mostly as an intramolecularly hydrogen bonded OH…N conformer (g′Gg′). On IR irradiation in matrices this conformer converts to NH…O “bonded” conformers. Photorotamerization occurs most efficiently after absorption of radiation corresponding to the νOH frequency. Radiation in the near-IR region has also been found to induce the same conformer changes. The IR-induced rotamerization is reversible, since selective irradiation at the νOH frequencies of the NH…O “bonded” conformers produces the intramolecularly hydrogen bonded OH…N conformer g′Gg′. The reverse reaction is 10–15 times slower than the forward reaction. A rapid temperature and irradiation dependent subprocess has been found in the nitrogen matrix. The conformers have been assigned with the aid of results obtained from ab initio calculations (minimal STO-3G basis) of fully optimized geometries of all the conformers. The possible photorotamerization paths are discussed with the aid of the calculated potential energy surface. On the basis of the calculations a trans-angle rule is presented for primary alcohols and amines.

Ab initio calculations on ethanol and 2-substituted fluorine and chlorine containing mono-, di- and tri-haloethanols, with reference to conformer interconversions at low temperatures

Abstract All conformers of ethanol and of several 2-substituted mono-,di- and trihalogenated (halogen = F and/or Cl) ethanols have been optimized, mostly at the 4–31G level. Semirigid torsional energy curves have been constructed for these compounds. For 2-fluoroethanol, calculations were carried out also using polarization functions, as well as on the MP and CID levels. The results are discussed with special reference to conformer interconversion processes in low-temperature matrices, and it is concluded that ab initio calculations help essentially in interpreting the experimental results. It is concluded that 4–31G is the most suitable level for calculating the torsional potential energy barriers for such processes (the minimal basis STO-3G giving much too high CO barriers and too low CC barriers). The CO barriers seem to be quite low for many conformational processes (or even non-existent); thus the only “stable” conformers at the low temperatures in question (10–30 K) seem to be Gg′ and Tt for monohaloethanols, Gg and Tg for 2,2-difluoroethanol and g for 2,2,2-trihaloethanols. The CC barrier between the Gg and Tg conformers of 2,2-difluoroethanol is quite high. The trans-angle rule proposed earlier is found to be independent of the basis set used.

IR‐induced photorotamerization of allyl alcohol in low‐temperature matrices and ab initio calculations

The infrared regions νOH, τOH, and 950–900 cm−1 of allyl alcohol (2‐propen‐l‐ol) were studied in low‐temperature Ar, Kr, Xe, N2, and CO matrices. A fast IR‐induced process and a similar slow dark process were found. It is concluded that the photoprocess is a rotamerization (obviously Cg→Gg′), since the process was similar in all five matrix media and also for other allyl compounds. The photoprocess was found to obey the first order rate law. Radiation from 9000 to below 2600 cm−1 was found to be active in initiating the process, especially the radiation corresponding to the νOH and methylenic νCH2 regions. The process is very fast in xenon. A possible mechanism for the process is discussed. To aid the interpretation of the results, ab initio calculations were carried out. All five conformers of allyl alcohol were optimized on the 4‐31G level, and the corresponding semirigid potential energy curves of the torsional coordinates were constructed. Some calculations were carried out also on the 6‐31G**, MP/4‐3...

IR-induced conformer interconversion processes of glycolaldehyde in low-temperature matrices, and ab initio calculations on the energetics and vibrational frequencies of the conformers

Abstract Both IR-induced and thermal conformer interconversion processes of glycolaldehyde (CHOCH 2 OH) were studied in low-temperature Ar, Kr and Xe matrices. The observed photochemical reaction induced by broad-band IR irradiation is interpreted to proceed from the lowest-energy conformer Cc which prevails after deposition to conformer Tt. In Xe and Kr the photochemical steady state (ratio [Tt]/[Cc]) depends on the matrix temperature; two conformers, Tg and Tt, seem to be in thermal equilibrium, and at elevated temperatures species Tg is enriched, and then the reverse photoprocess Tg → Cc occurs more rapidly than the competing reverse process, Tt → Cc which prevails at 13 K. Because of the thermal equilibrium the routes of the forward and reverse photoprocesses are considered to be different. A thermal reverse process, Tt → Tg → Cc, is observed at temperatures above 30 K in Xe and Kr. The role of the torsional barrier heights in determining the rates of the photoprocesses is emphasized. To aid in the interpretation of the experimental results, ab initio calculations were carried out. Geometries and energies of all conformers of glycoaldehyde as well as the geometries of the most important saddle points were fully optimized at the HF/6-31G** level, and vibrational spectra were calculated on the HF/4-31G level; the latter conformed to the experimental conformer assignment.

IR-induced rotamerization of 1-propanol in low-temperature matrices, and ab initio calculations

Abstract IR-induced conformer interconversion of 1-propanol was studied in solid Ar, Kr, Xe and N 2 matrices, and was found to involve a CC rotation around the CH 2 CH 2 bond. Ab initio calculations were carried out to aid the interpretation of the results. The photoprocess was found to obey a first-order rate law under unfiltered Nernst-glower irradiation and the rate in different matrix media decreased in the order Xe > Kr > Ar > N 2 . In Xe matrices the rate depended on the deposition temperature, the process being ≈ 3 times faster when the deposition temperature was 15·K than when the sample was deposited at 25 K. A thermal conformer interconversion similar to the photoprocess was found in Xe above 40 K. The efficiency of different spectral regions in initiating the photoprocess was studied using several interference filters.

IR and UV induced photorotamerization of 2‐nitroethanol in matrices. Ab initio optimization of conformers and potential energy calculations

Conformer interconversion of 2‐nitroethanol was studied in rare gases Ar, Kr, and Xe as well as in N2 matrices. The initial conformer Gg′ is transformed into conformer Tt both by UV (selectively with wavelengths of about 280 nm) and by IR irradiation. Frequencies in the νOH region were found to be the most effective in initiating the IR induced photorotamerization. The process is reversible, i.e., irradiation at the νOH band frequencies of the product conformer Tt produces the conformer Gg′. The IR induced process obeys the first order rate law in all the matrices studied, and the rate in different hosts decreases in the order Xe≫Kr≳Ar≳N2. Upon prolonged broadband IR irradiation the process proceeds to an equilibrium; the final Tt/Gg′ conformer ratio (determined from the νOH bands) was the same, ∼0.5, in all matrices, while upon UV irradiation in Ar the final ratio was ∼1.1. The observation that in a Xe matrix at deposition temperatures above 20 K the rotamerization is almost totally inhibited, is explain...

Infrared-induced rotamerizations in low-temperature matrices

Abstract We have observed IR-induced conformer interconversions for several 2-substituted ethanols, propanols, allylic compounds and amines in low-temperature matrices. Besides, we have found many similar compounds which do not rotamerize under our experimental conditions, and possible reasons for this are discussed. The photorotamerization processes which we have studied seem always to obey the first order rate law under the unfiltered Nernst glower irradiation, and the rate of the photoprocess is strongly medium-dependent. The rate in different matrix media decreases in the order Xe > Kr > Ar > N 2 .

Ab initio calculations on hono: Energies, geometries and force fields on different levels of theory

Abstract The geometries of the electronic ground states of cis and trans HONO have been optimized on several levels of theory, and the frequencies and force fields calculated on 5 different levels (including HF, MP2 and CID calculations). The force fields were reduced and then averaged. An estimate of 1302 cm−1 was obtained for the wavenumber of the HON bending band of cis HONO. The geometry of the torsional maximum was optimized on a few levels; its frequencies were calculated using the basis 4-31G. The trans—cis torsional barrier height is estimated as 44 kJ mol−1 (the activation energy of this process being 39 kJ mol−1). Improving the level of calculation did not automatically lead to improved values for the cis—trans energy difference. The possibility of an intramolecular interaction in cis HONO is discussed.

Ab initio calculations on 2-fluoroethanol and 2-chloroethanol with reference to the infrared photorotamerization processes in low-temperature matrices

Abstract Ab initio STO-3G optimizations have been carried out on the five conformers of 2fluoro- and 2-chloro-ethanol and on ten saddle point conformations of 2-fluoroethanol. The relative order of the calculated conformer energies is found to be Gg′ ⪡ Tt −1 , respectively. Large variations (4–7°) are found for the angles CCO and OCH from conformer to conformer and a general angle rule is presented for primary alcohols. Neglect of this effect is the main source of error in rigid-rotation calculations on alcohols (as compared with fully optimized calculations), of the order of 5 kJ mol −1 . The use of calculated OH stretching force constants in assigning νOH band fine structure in IR spectra is discussed. Evidence for a weak intramolecular hydrogen-bonding interaction is found, especially in the case of 2-chloroethanol. On the basis of calculated conformer energies, barriers to rotation and force constants it is deduced that for both of these alcohols the photorotamerization process in matrices is either Gg′ → Tt or Gg′ → Gt.

Thermal and infrared-induced conformer interconversion processes for 2-fluoroethylamine in solid Xe

Abstract Four conformers, Gg, Gt, Tg and Tt, of 2-fluoroethylamine were found to be present in Xe matrices at temperatures between 15 and 65 K. Two parallel CC rotations, Gg → Tg and Gt → Tt, were induced by infrared irradiation. At temperatures above 35 K reverse thermal processes occurred. The species Gg and Gt are at thermal equilibrium at temperatures above ≈33 K and their energy difference is ≈400 J mol −1 .