Gad Fischer

Molecular beam infrared spectra of dimers formed from acetylene, methyl acetylene, and ethene as a function of source pressure and concentration

Infrared spectra have been obtained for dimers formed from acetylene, methyl acetylene, and ethylene using a wide variety of free jet source conditions. An F‐center laser was used to excite various C–H stretch fundamentals in the range 3000–3300 cm−1. Rotational structure was resolved in the spectra of both acetylene and methyl acetylene dimers when dilute mixtures of either gas in helium were expanded from high pressures. The spectrum of the ethylene dimer showed no such structure even when formed in very dilute mixtures. These results contradict previous suggestions that the vibrational relaxation lifetime is uniformly short for all polyatomic dimers.

New infrared spectra and the tautomeric studies of purine and αl-alanine with an innovative sampling technique

Abstract New infrared spectra of purine and α l -alanine in the mid-IR range (400–4000 cm−1) were obtained with an innovative infrared sampling technique. The technique was directed at maximizing the separation of solid sample molecules at room temperature through a procedure of dissolution, spray, and deposition. The notoriously broad and overlapped bands in the region 2500–3300 cm−1, which are characteristic of the KBr pellet spectra of amino acids and nucleic acid bases, are now absent in all our new spectra. The spectra obtained from neutral and acidic aqueous solutions are different. For purine, these spectral differences are interpreted in terms of the existence of N(7)H or N(9)H tautomers; For alanine, the differences are ascribed to its two different structures, zwitterion and un-ionized neutral. It was found that with this new method, better resolved IR spectra of purine and alanine were obtained at room temperature. Furthermore, for the first time, a spectrum of zwitterionic alanine of largely monomeric form in the complete mid-IR range was obtained. IR spectra were predicted from self consistent reaction field (SCRF) molecular orbital calculations at the HF/6-311++G(d, p) level. The SCRF calculations were for purine and alanine present in a KBr continuum. Good agreement was found between the observed and calculated spectra.

Jahn‐Teller Distortion of s‐Triazine in Its Lowest Excited Singlet State

An assignment of the lowest excited singlet state of s‐triazine as 1E″, nπ*, is made based on the vapor and polarized crystal absorption spectra of both the perproto and perdeutero isotopes. A theory of vibronic intensities is presented for the case in which one and the same vibrational motion can both derive intensity for a forbidden doubly degenerate state and remove the orbital degeneracy. Spectral results are presented which indicate that s‐triazine undergoes a Jahn‐Teller distortion in the 1E″ state.

The 2670 Å absorption system of formyl fluoride

Abstract A new analysis of the 2670 A absorption spectrum of formyl fluoride is presented. Many temperature dependent bands are observed for the first time. The electronic origins of HCOF and DCOF are located at 37 488 and 37 504 cm −1 . The spectrum is assigned to a transition between a planar ground state 1 A ′, of symmetry group C s , and a pyramidal excited state 1 A , of symmetry group C 1 , with the out-of-plane angle between the CO axis and the HCF plane in the range 30 to 35°. Inversion doubling leads to measurable splitting for bands with 2, 3 (1 + , 1 − ) and 4, 5 (2 + , 2 − ) quanta of the CH out-of-plane bending mode, ν 6′ , excited. Transitions from the vibrationally excited ground state give the first direct experimental measurement of ν δ″ for HCOF, 1010 cm −1 .

Vibrational predissociation of Van der Waals clusters of ethylene

Abstract Infrared laser spectroscopy and quadrupole mass spectrometry have been used to study clusters of ethylene molecules formed in a free jet expansion. Vibrational spectra in the range 2950—3200 cm −1 show structure corresponding to the predissociation of clusters as a result of exciting them near ν 1 -ν 11 and r u fundamentals, as well as the ν 2 + r 12 combination band. These results provide valuable information concerning the structures and dissociation mechanisms for ethylene clusters. In all cases the vibrational bands show lorentzian lineshapes of = 5 cm −1 fwhm, suggesting that the dominant broadening mechanism is homogeneous. Estimates of the excited-state lifetimes can therefore be obtained directly from these widths.

Assignment of the Lowest Singlet and Triplet States of the Monoaldehyde of Durene

Abstract The monoaldehyde of durene has been synthesized. It has been shown to be the impurity frequently encountered in spectral studies involving durene crystals. Both the polarized absorption and emission spectra of the protonated and deuterated aldehyde in crystals of dureneh-14 and durened-14 have been obtained. Although the lowest excited singlet and triplet states, separated by 2200 cm−1, are shown to be nπ∗ a number of features in the phosphorescence spectrum are more characteristic of a 3π∗ state. The polarization of the absorption spectrum, in particular the 0-0 band and bands involving the excitation of totally symmetric vibrations, is inconsistent with the nπ∗ nature of the transition. A number of explanations is proposed.

The infrared spectra and molecular structure of zwitterionic l-β-phenylalanine

Abstract The infrared absorption spectrum and molecular structure of zwitterionic l -β-phenylalanine (Phe) were studied by means of a new sampling technique and ab initio molecular orbital calculations. The self-consistent reaction field calculations at HF/6-311G(d,p) level were carried out on zwitterionic Phe present in a continuum of KBr. Good agreement in terms of both frequencies and intensities was found between the calculated and observed full mid-IR spectra. The predicted structure of monomeric zwitterionic Phe was discussed and compared with Phe analogues.

Line shapes in electronic absorption spectra. Phenanthrene

Abstract The 3000 A, ( 1 B 2 ← 1 A 1 ), absorption system of phenanthrene in durene crystals at 4°K illustrates an electronic transition, which is subject to near-resonance vibronic perturbations whose effect is intermediate to both the small (sparse intermediate) and large molecule (statistical) limits. Both broad (300 cm −1 ) and narrow (10 cm −1 ) lines are evident. A model is proposed which incorporates both these features by fast allowing for a consideration of the interaction between a small number of discrete levels, those associated with the largest coupling, followed by a treatment of the broadening of these levels through interaction with the remaining near continuum of states of the lower electronic state. Thus, one and the same electronic state provides both a sparse and dense manifold of levels. An important result of the model is that in terms of absorption intensities all the lines emerge with the same heights but differ in widths. When the intensities are summed with respect to energies this aspect is obscured. This approach has been shown to satisfactorily reproduce many of the features of the 1 B 2 absorption spectra of phenanthrene and phenanthrene- d 10 . The 1 B 2 absorption systems have also been measured in the vapour phase and fine structure attributable to vibronic coupling and sequence band development are discussed.

Electronic spectra of carbon chain anions: C2nH− (n=5–12)

The electronic absorption spectra of mono-hydrogenated carbon chain anions C2nH− (n=5–10) have been measured in the gas-phase and in 6 K neon matrices (n=8–12). The techniques of resonant two-color electron photodetachment in the gas-phase and absorption spectroscopy of mass-selected anions in neon matrix were used. A homologous series is observed, with band system origins shifting from 304 nm for C10H− to 590 nm for C20H−. In conjunction with ab initio calculations the band systems are attributed to a 1Σ+←X 1Σ+ transition of linear acetylenic anions. Another near lying electronic transition due to a second isomer is also apparent for C10H− up to C24H−. Comparison with tables of the known diffuse interstellar bands indicates possible matches for the origin bands of the C18H− and C20H− isomers.

Infrared absorption spectra and structures of zwitterions of glycyl-L-alanine and its N-deuterated isotopomer in a KBr matrix

Abstract The IR absorption spectra of zwitterions of glycyl- l -alanine and its N -deuterated isotopomer isolated in a KBr matrix have been measured using a novel sampling technique involving dissolution, spray and deposition. In a parallel theoretical study the number of conformers, their IR absorption spectra and structures have been determined by ab initio molecular orbital calculations. The calculations were carried out at both the density functional theory and Hartree–Fock levels, and were of the non-aqueous self-consistent reaction field type, using the Onsager dipole-sphere model. Four conformers were identified. The trans conformer was found to have the lowest energy at all levels of theory and for the range of cavity radii studied. Its structure was found to be closely similar to that of glycyl- l -alanine in the crystal. Comparison of the predicted spectra of the four conformers with the measured spectra showed that both the trans and/or cis conformers could be present in the KBr matrix. Good agreement between measured and calculated spectra, both cis and trans , has been obtained for the majority of the prominent bands in the spectrum.