Timo Lotta

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...

Characterization of Langmuir-Blodgett films of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphatidylcholine by FTIR-ATR

Monomolecular films of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphatidylc holine (PPDPC) were transferred from an air/water interface onto a germanium attenuated total reflection crystal by the Langmuir-Blodgett (LB) technique. The assemblies were thereafter investigated by Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy. To determine the molecular organization in the deposited layers we monitored the CH2 and C = O stretching and the CH2 bending regions of the infrared spectra of these lipids in detail. Using Fourier self-deconvolution technique, the carbonyl stretching mode was resolved into two models corresponding to the conformational differences in the ester linkages of the phospholipid sn-1 and sn-2 acyl chains. By varying the temperature of the subphase and using different surface pressures, we were able to transfer different conformational states of DPPC onto a germanium ATR crystal. Deposition of DPPC at 40 mN m-1 and at 15 degrees C or at 20 mN m-1 and at 35 degrees C results in LB-assemblies in ordered or disordered states, respectively, as judged by the IR spectra. These structures in LB films correspond to the state of DPPC in liposomes below and above the temperature of the order-disorder phase transition. Irrespective of the surface pressure and subphase temperature used during the deposition, an ordering process was found in DPPC films when the number of the transferred layers was increased from one to five. The pyrene-labelled phosphatidylcholine analogue, PPDPC, behaved differently from DPPC. In the case where one to three layers of PPDPC transferred at 35 mN m-1 and at 20 degrees C only conformational structures resembling those in fully hydrated liposomes above the main transition temperature were observed.

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.

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.

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 .

Thermal and IR‐induced conformer interconversion processes for 2‐fluoroethylamine in several low‐temperature matrix media and ab initio 4‐31G, 4‐31G(N*), and 6‐31G** calculations

IR‐induced conformer interconversion processes have been found to occur for 2‐fluoroethylamine in solid Ar, Kr, Xe, and N2 matrices but not in solid CH4. Using information obtained from the photoprocesses as well as from thermal processes and ab initio calculations, it was possible to find out the conformers present, to evaluate the conformer energies and torsional barrier heights, and to unravel the occurring processes. The total IR process consists of two parallel first‐order C–C rotations, Gg→Tg and Gt→Tt. The influence of the host molecules upon the rates and barrier heights, the efficiency of different spectral regions in initiating the process, and the thermal processes were studied. The latter occur as C–C rotations opposite to the photoprocesses and as conformational equilibria involving rotations around the C–N bond. Extensive ab initio calculations were carried out on the 4‐31G, 4‐31G(N*), and 6‐31G** levels. Geometries of the conformers were fully optimized at these levels, and torsional potent...

Fourier transform infrared study on the thermotropic behaviour of fully hydrated 1-palmitoyl-2-[10-(pyren-1-y) decanoyl]-sn-glycero-3-phosphatidylcholine.

Fourier transform infrared (FTIR) spectroscopy was used to study the thermotropic behaviour of fully hydrated 1-palmitoyl-2-[10-(pyren-1-yl)-decanoyl]-sn-glycero-3-phosphatidyl choline (PPDPC) in the temperature range of 3-30 degrees C. Several changes in the spectral features of PPDPC were observed. Major alterations analogous to the gel-to-liquid crystalline phase transition of saturated phosphatidylcholines were evident at approximately 16 degrees C in both the wavenumbers and the halfbandwidths of five different vibrational modes of PPDPC, viz. asymmetric and symmetric CH2 stretching, C = O stretching, and CH2 bending. Also the pyrene ring deformation mode changed at this temperature. Using Fourier self-deconvolution technique we resolved the carbonyl stretching mode into two bands at approx. 1741 and 1726 cm-1. These bands are due to conformational differences in the ester linkages of the two acyl chains, and are further assigned on the basis of literature data to the sn-1 and sn-2 carbonyl groups, respectively. The ratio of the relative intensities of these two bands is shown to depend on the phase state of the phospholipid.

Thermal and IR-induced conformer interconversion processes for 2-chloroallyl alcohol in Low-temperature matrices: Wavelength-dependent conformer steady states

Abstract 2-chloroallyl alcohol has been studied in low-temperature Ar, Kr, Xe, N 2 and CH 4 matrices. An IR-induced process was found in all media except CH 4 and resulted in a steady state dependent on the Wavelength of the irradiation. At elevated temperatures thermal conformer interconversions occurred. The energetics of the processes are discussed and their potential barriers determined. No mode selectivity was found.