Benuel J. Kelsall

Infrared spectra of the hydrogen‐bonded pi complex C2H4–HF in solid argon

The hydrogen‐bonded pi complex C2H4‐‐HF has been prepared by codeposition of C2H4 and HF with excess argon at 12 K. Isotopic substitution in both submolecules identified the complex and provided assignments to νs at 3731 cm−1, νl at 424 and 396 cm−1, and ν7c at 973 cm−1. The observation of a substantial perturbation on the out‐of‐plane bending mode of the C2H4 submolecule in the complex without a perturbation on the in‐plane bending mode indicates that H–F is bonded to the pi electron system perpendicular to the plane of the molecule. The positions of νs and νl indicate a hydrogen bond of intermediate strength and the splitting in νl is characteristic of the anisotropy in the hydrogen bonding interaction with a π system.

Two‐color resonance photoionization of aromatic molecules in solid argon

High pressure mercury arc photolysis of argon matrix samples of biphenyl, naphthalene, and 1‐phenyl‐1‐propyne containing CCl4 for an electron trap produced very strong parent radical cation absorptions. Filtered irradiations show that two‐color photoionization is responsible for ion formation in these experiments. In the case of biphenyl and naphthalene, photolysis data show that the relatively long‐lived S1 states are the intermediate in the two‐color photoionization process, whereas in the case of 1‐phenyl‐1‐propyne, the T1 state contributes to the two‐color photoionization process. The observation of two‐color photoionization in these experiments suggests that resonant absorption by an excited molecule into ionization is an extremely high cross‐section process.

FTIR observation of the N2‐‐‐HF complex in solid argon

Sharp absorptions at 3881.5 and 262 cm−1 in argon matrices containing HF are shown to depend upon N2. Substitution with DF indicates assignments to the νs and νl modes of the linear N2‐‐‐HF complex.

FTIR spectroscopic studies of the matrix photoionization and photolysis products of methylene halides

Abstract Matrix photoionization of methylene bromide produced absorptions at 1019, 897, and 788 cm −1 identified previously as CBr 2 + , CHBr 2 + , and CHBr 2 . High-resolution FTIR spectra revealed overlapping 1/2/1 triplets for natural bromine isotopes with individual linewidths near 0.2 cm −1 . New absorptions at 3121, 2897, and 1345 cm −1 are assigned to the (CH 2 Br + )Br cation complex which yields CHBr 2 + on photolysis. A substantially increased yield of the CHCl 2 + species made possible observation of the CH stretching mode at 3033 cm −1 and the symmetric CCl 2 stretching mode at 845 cm −1 along with the previously observed stronger 1291- and 1044-cm −1 fundamentals. The high resolution and enhanced signal-to-noise capability of the FTIR are clearly demonstrated in this investigation.

Matrix photolysis of (C6H5)2CH2, (C6H5)2CHBr, (C6H5)2CHCl and (C6H5)2CCH2. Visible and ultraviolet spectra of the transient products

Abstract Absorption spectra (1400−300 nm) of (C 6 H 5 ) 2 CH 2 , (C 6 H 5 ) 2 CHBr, (C 6 H 5 ) 2 CHCl and (C 6 H 5 ) 2 CCH 2 samples in argon matrices subjected to argon resonance radiation are described. The three diphenylmethane compounds give a sharp band at 327 nm due to the diphenylmethyl radical. The chloride and bromide give a broad band at 448 nm, which is assigned to the daughter cation (C 6 H 5 ) 2 CH + . The chloride gives an additional band at 567 nm, which decreases on photolysis and a new maximum forms at 556 nm; these bands are believed to be due to isomeric parent cation structures. The diphenylethylene precursor gives a 326 nm band for the methyl diphenylmethyl radical and photosensitive bands at 1280 and 367 nm for the parent cation. Addition of hydrogen to the discharge produces a new 433 nm band for (C 6 H 5 ) 2 (CH 3 )C + at the expense of the parent cation.

Vibronic absorption spectra of phenyl alkyne cations in solid argon at 20 K

Phenyl alkyne samples at high dilution in argon were subjected to argon resonance radiation during condensation at 20 K. Sharp band origins at 579.0 and 586.0 nm agree with the differences between sharp first and fourth photoelectron bands for phenyl acetylene and 1‐phenyl‐1‐propyne, which identifies the molecular cation absorptions. Vibronic assignments are supported by isotopic substitution and comparison to spectra of phenyl acetylene. Substitutent vibrations dominate the red transition, which is predominantly π(ring) ← π(alkyne), whereas ring vibrations dominate the UV transition for 1‐phenyl‐1‐propyne, which is localized on the aromatic ring.

Absorption spectrum of the carbon tetrachloride molecular cation in noble-gas matrices

Carbon tetrachloride diluted in argon, krypton or xenon and subjected to vacuum-ultraviolet radiation during and after condensation gave strong photosensitive absorptions at 425, 430 and 455 nm, respectively, which are assigned to CCl4+. This study suggests that absorptions in this region following radiolysis of CCl4 in hydrocarbon glasses are probably due to CCl4+ as well and that structural relaxation follows ionization of CCl4 in the solid phase.

Absorption Spectra of Diphenylacetylene and 1,4-Diphenylbutadiyne Cations in Solid Argon

One and two-photon matrix photoionization techniques have been used to prepare diphenylacetylene and 1,4-diphenylbutadiyne cations. Identification of these cations was confirmed by photoelectron spectra and radiolysis studies. Near-infrared absorption bands for each cation correlate with photoelectron spectra and suggest structural relaxation about the molecular axis on ionization, and strong blue absorptions that do not correlate with photoelectron spectra are probably caused by π → π* transitions.

Fourier Transform Infrared Matrix Isolation Spectroscopy

Low temperature matrix isolation spectroscopy has been developed over the past two decades into a powerful technique for the study of reactive chemical species. When reactive molecules, radicals or ions are trapped in low concentrations in a frozen film of an unreactive material such as argon, they may be studied spectroscopically without the use of rapid scanning instrumentation. This technique is especially suited to the study of vibrational spectra of trapped species. The sharp vibrational absorptions observed in the absence of complicated rotational structure are well suited for study by high-resolution Fourier-transform infrared spectroscopy, especially in studying compounds containing natural abundance or enriched isotopes. High resolution FTIR spectroscopy has been used recently in this laboratory to observe natural abundance chlorine isotope splittings in the IR spectra of ions produced by illuminating matrix isolated methylene chloride with 11 eV photons. Isotopic effects on complexes of hydrogen fluoride and ethylene in solid argon have also been successfully studied using high resolution FTIR techniques.