J. Nieminen

The mechanism of formation and infrared-induced decomposition of HXeI in solid Xe

Ultraviolet (UV) irradiation of HI-doped xenon matrix dissociates the precursor and leads to the formation and trapping of neutral atoms. After UV photolysis, annealing of the matrix mobilizes the hydrogen atoms at about 38 K. The mobilized hydrogen atoms react with I/Xe centers forming HXeI molecules in a diffusion controlled reaction. The formed molecules can be photolyzed with infrared (IR) irradiation at 2950–3800 cm−1 and quantitatively regenerated thermally. The formation of HXeI from neutral atoms is proved by the quantitative correlation between neutral iodine atoms and HXeI molecules in selective IR photodissociation and thermal regeneration experiments. Kinetic measurements show that the formation of HXeI from atoms is prevented by a potential barrier, which is estimated to be 700 cm−1 in magnitude. The potential barrier is proposed to originate from the avoided crossing between neutral H+Xe+I and ionic (HXe)++I− singlet surfaces. The dissociation energy D0 of HXeI with respect to the top of the...

Rotational Isomerism in Acrylic Acid: A Combined Matrix-isolated IR, Raman and ab initio Molecular Orbital Study

The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. In addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the CC—CO axis. Upon irradiation at λ= 243 nm by a xenon lamp, the s-cis form (CC—CO dihedral angle equal to 0 °), corresponding to the conformational ground state, converts to the s-trans form (CC—CO dihedral angle equal to 180 °). In the liquid phase, dimeric structures strongly predominate, but the existence in this phase of the two conformational states referred to above can also be inferred from the corresponding vibrational spectra. In turn, in the crystal only the thermodynamically most stable form (s-cis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of acrylic acid are also presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical harmonic force fields and of IR intensity studies based on the charge–charge flux–overlap (CCFO) model were used to help interpret the experimental vibrational data, enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.

Potential energy surface and vibrational analysis along the stretching vibrations of the ArHAr+ ion

Abstract An ab initio calculated potential energy surface along the stretching coordinates of the linear ArHAr + ion presented. The potential surface has been obtained using the MP2/15s11p3d1f/7s2p1d level of the ab initio electronic structure calculation theory. Qualitatively the potential is characterised by a development of double minimum structure at elongated Ar---Ar distances at higher vibrational energies and by a large amplitude for the motion of the hydrogen atom. The stretching vibrational energy levels and wavefunctions have been calculated variationally using harmonic oscillator basis functions. The vibrational analysis indicated extensive mixing of the zeroth-order harmonic oscillator vibrational states already at the bottom of the well giving rise to strong absorption “progression” starting at 1208 cm −1 . The Einstein A coefficients for spontaneous emission transitions have also been calculated and the IR emission spectrum is predicted to be particularly rich in the 1100–1900 cm −1 spectral region.

Potential energy surface and vibrational analysis along the stretching vibrations of XeHXe+ ion

An analytical potential energy surface (PES) along the stretching coordinates of a linear XeHXe+ ion is presented. Ab initio calculations within the effective core potential approach are used as input for the PES. The present vibrational analysis indicates extensive mixing of the zeroth‐order harmonic oscillator vibrational states, and a rather complete collapse of the normal mode picture already near the bottom of the potential well. At higher vibrational energies, and elongated Xe–Xe distances, development of a double minimum in the PES is observed. The simulated absorption spectrum consists of a strong vibrational progression near 1000–1700 cm−1, and is in qualitative agreement with the previous matrix isolation data. The intensity distribution of the vibrational progression is mostly due to the potential terms rather than nonlinear contributions in the Taylor series expansion of the electric dipole moment. Due to the highly anharmonic potential, and subsequent breakdown of selection rules, the emissio...

Spin–orbit transitions (2P1/2←2P3/2) of iodine and bromine atoms in solid rare-gases

Abstract Infrared absorptions of iodine and bromine atoms in solid rare-gases are presented. The isolated atoms are produced by UV-photolysis of HI or HBr in solid Ar, Kr and Xe. The absorptions are characterised by sharp zero-phonon lines and broad structured phonon side bands. Some of the zero-phonon lines are resolved and split into two components, separated by a few wavenumbers. The production of the atoms follow second-order kinetics indicating that the primary hydrogen atom undergoes a secondary reaction with the hydrogen halide producing a halogen atom and a hydrogen molecule. This is supported by the observation of the infrared absorptions of hydrogen molecules in concentrated matrices after photolysis.

All-electron and effective core potential studies on ground state ArH+, KrH+ and XeH+ ions

Results of extensive all-electron and effective core potential (ECP) ab initio calculations on the ground state properties of ArH+, KrH+ and XeH+ are reported. It is demonstrated that, for the equilibrium distance and harmonic frequency, the ECPs combined with a hydrogen basis set of sufficient size and flexibility lead to an accuracy quite similar to the most sophisticated all-electron treatments. The + VPs(2s)2 hydrogen basis by Latajka and Scheiner is shown to be particularly suitable in this regard. The obtained results are rather promising with respect to future calculations on larger ionic clusters.

Anomalous temperature dependence of the IR spectrum of polyalanine

Abstract We have studied the temperature dependence of the infrared spectra of acetanilide (ACN), tryptophan–(alanine) 15 , and tyrosine–(alanine) 15 . No sidebands of the amide-I vibration were observed in the polypeptides, but two anomalous sidebands of the NH stretch with a similar temperature dependence as that of the anomalous amide-I vibrational mode at 1650 cm −1 of crystalline ACN were detected. Fermi resonance combined with the appearance of a red-shifted sideband of NH stretch through coupling to lattice modes seems to explain this band structure. Observations are indicative of excitons that may occur in polypeptides as well as in single crystals of ACN.

Matrix isolation infrared and ab initio studies on conformers of 3-fluoropropene

Abstract The infrared spectra of the cis and gauche conformers of 3-fluoropropene, CH 2 CHCH 2 F, were studied in Ne, Ar, Kr and Xe matrices. An infrared-induced cis to gauche rotamerization was found in Ar, Kr and Xe matrices. A thermal interconversion process was also found. Its direction was dependent upon the host, being the same as that of the IR process in Kr but reverse in Ar and Xe. In Ar and Xe matrices considerable site-splitting occurs in the IR spectra and a detailed analysis of the processes in different sites is given. An energy difference of 2.5±0.3 kJ mol −1 between the cis and gauche species was obtained on assuming that the gas phase equilibrium between the conformers is trapped upon deposition. A slow dark process from cis to gauche conformer was observed in Kr matrices at temperatures above 15 K, possibly due to tunnelling. Ab initio calculations were carried out on 3-fluoropropene. The torsional potential energy curve and spectra of the conformers were calculated at the MP2(full)16-31G** level and were compared with the experimental results.

Excitation Transport in Helical Proteins

Recent results for excitation dynamics in and IR-absorption spectrum of helical polypeptides are briefly reviewed.