Author Institution: Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441

When a sample of neon to which have been added less than 1% each of H-2 and H2O is deposited at 4.3 K, the infrared spectrum of the resulting solid includes an absorption by the vibrational fundamental of H-2, which is normally infrared inactive. New absorptions are also associated with the vibrational fundamentals of the H2O in the sample. Similar results are obtained for deuterium-enriched samples. The new peaks are assigned to the van der Waals complex of H2O with H-2. As has been found in earlier theoretical, gas-phase, and solid-state studies of this and closely related systems, the infrared absorptions arise principally from complexes involving ortho-H-2, for which J=1.

When a sample of neon to which have been added less than 1% each of H2 and H2O is deposited at 4.3 K, the infrared spectrum of the resulting solid includes an absorption by the vibrational fundamental of H2, which is normally infrared inactive. New absorptions are also associated with the vibrational fundamentals of the H2O in the sample. Similar results are obtained for deuterium-enriched samples. The new peaks are assigned to the van der Waals complex of H2O with H2. As has been found in earlier theoretical, gas-phase, and solid-state studies of this and closely related systems, the infrared absorptions arise principally from complexes involving ortho-H2, for which J=1.

When a Ne:HCOOH sample is codeposited at ca. 5 K with neon atoms that have been passed through a microwave discharge, new absorptions appear in the infrared spectrum of the resulting solid that can be assigned to trans-HOCO, trans-HCOOH+, and HCO2-. The absorptions of trans-HOCO are readily identified by a comparison with those previously reported for that molecule trapped in solid argon. Preliminary assignments of infrared absorptions of HOCO+, confirmed in studies using another experimental system, are also suggested. The identifications of trans-HCOOH+ and of HCO2- are aided by study of the photodestruction characteristics of these products when the deposit is exposed to various wavelengths of visible and ultraviolet radiation, by an analysis of the spectra obtained from isotopically substituted samples, and by a comparison with the results of ab initio and density functional calculations. Three previously unidentified vibrational fundamentals of trans-HCOOH+ have been assigned, as have been four vibrational fundamentals of HCO2-, in the inert, nonionic environment of solid neon. The CH-stretching fundamental of HCO2- appears at an exceptionally low frequency. The results of density functional calculations of the structures and vibrational fundamentals of trans-HCOOH+, HCO2-, the W(C-2v) structure of C(OH)(2)(+), and cis- and trans-HCOOH- are given.

When a Ne:allene or a Ne:propyne sample was codeposited at approximately 5 K with a sample of pure neon that had been excited in a microwave discharge to provide a 16.6–16.85 eV energy source, prominent new infrared absorptions which can be assigned to the H2CCCH2+ cation appeared. Also present in the allene experiments were two absorptions which can be assigned to H2CCCH−. In the propyne experiments, an absorption is tentatively attributed to the strongest infrared fundamental of CH3CCH+. The structures and vibrational fundamentals obtained from density functional and ab initio calculations for various isotopomers of H2CCCH2+, CH3CCH+, cyc-C3H3+, H2CCCH+, and H2CCCH− are given. The infrared absorption pattern of the cation common to the allene and propyne experiments matches that of the predominant gas-phase product, cyc-C3H3+, reasonably well, except for the presence of extra peaks in the CH-stretching region. However, comparison of the results of experiments on isotopically substituted samples with the calculated spectra excludes that assignment and supports the identification of the neon-matrix product as H2CCCH2+. It is suggested that collisions with the excess of neon atoms in the sampling region rapidly remove excess energy from the initially formed allene and propyne cations, inhibiting the loss of an H atom from those two species.

Author Institution: National Institute of Standards and Technology; Optical Technology Division, National Institute of Standards and Technology

When a Ne:SO2 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned with the aid of isotopic substitution studies to SO, SO2+, SO2−, (SO2)2−, and, tentatively, SO−. The fundamental and first overtone absorptions of SO lie 0.9 and 1.8 cm−1, respectively, below the gas-phase band centers. Ab initio calculations at the Hartree–Fock level show an instability in the v3 vibration of SO2+ which is avoided by higher-level calculations. The ν3 and ν1 fundamentals of SO2− isolated in solid neon are identified at 1086.2 and 990.8 cm−1, respectively. In agreement with an earlier proposal, the 1042 cm−1 absorption originally assigned to ν3 of SO2− trapped in solid argon is reassigned to MSO2, with M an alkali metal. Near the photodetachment threshold for SO2− isolated in a neon matrix, electron capture by SO2 nearest-neighbor pairs results in growth of infrared absorptions of (SO2)2−, which has been shown by gas-phase studies to have a significantly higher photodetachment threshold than does SO2−. The isotopic substitution studies require that the two sulfur atoms in (SO2)2− be nonequivalent, favoring the linking of the two SO2 units by a S ⋯ O bond.

A number of molecular ions have been stabilized in solid neon in sufficient concentration for detection of their infrared spectra. The neon matrices were prepared by codepositing the ion precursor, diluted in an excess of neon, at approximately 5K with a beam of excited neon atoms. The results of these experiments are surveyed, and the studies of the hydrogen halides (X=Cl, Br, I) and methyl halides (X=F, Cl, Br) are described in greater detail. The position of the ν2+ absorption of uncharged (HX)2 is exceptionally sensitive to the matrix material. Rotation of simple hydrides, including H2O, HX, and CH3, in solid neon is inhibited by the electric field associated with the presence of ions. The fundamental absorptions of HX+ in the neon matrix lie near the gas-phase band centers. Absorptions of (HX)2+ are also identified, and the XHX− anion contributes to the infrared spectrum. In studies of the methyl halides, infrared absorptions of both the conventional (CH3X+) and the ylidion (H2CXH+) isomers are identified and assigned with the aid of ab initio calculations. The conventional structure is significantly distorted from threefold symmetry by Jahn–Teller interaction.

Absorption spectra showing transitions to several excited electronic states of C 2n+1 (n ) 2-5) chains were observed. The spectra were recorded after codeposition of mass-selected anions with neon to form a matrix at 5 K. The assignment follows from mass-selection, the monotonic dependence of the wavelength of the electronic origin band on the number of carbon atoms, photobleaching changes, and spectroscopic considerations. The absorption band systems can be attributed to 2Π r X 2Π transitions. For C5 two transitions are observed, for C 7 three, and for C9 and C11 four.

When a Ne:CH(3)X (X=F, Cl, Br) sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit includes prominent absorptions which can be assigned to cation products. It has previously been established that the ylidion (H(2)CXH(+)) isomers have stable potential minima and that H2CFH+ is lower in energy than CH3F+. The identification of the new absorptions is aided by experimental studies on isotopically substituted Ne:CH(3)X samples and by ab initio calculations of the structures and vibrational fundamentals of both CH(3)X(+) and H(2)CXH(+). In each of the three systems, a prominent absorption which is intermediate in frequency between the gas-phase (and neon-matrix) absorptions of HX and HX(+) can be assigned to the ylidion, as can be several other absorptions. Still other absorptions can be assigned to CH3Cl+ and CH3Br+. The behavior of the product absorptions on exposure of the deposit to filtered visible and ultraviolet radiation is consistent with the proposed assignments.

Absorption spectra showing transitions to several excited electronic states of C-2n+1(-) (n = 2-5) chains were observed. The spectra were recorded after codeposition of mass-selected anions with neon to form a matrix at 5 K. The assignment follows from mass-selection, the monotonic dependence of the wavelength of the electronic origin band on the number of carbon atoms, photobleaching changes, and spectroscopic considerations. The absorption band systems can be attributed to (2) Pi <-- X (2) Pi. transitions. For C-5(-) two transitions are observed, for C-7(-) three, and for C-9(-) and C-11(-) four.