Matrix-isolation spectroscopy in solid parahydrogen is a powerful technique for the study of photochemical reactions in detail. It is especially suited for the detection of chemical intermediates and unstable molecular products of photochemical reactions that are otherwise difficult to observe using other conventional spectroscopic techniques. Here, we have revisited UV photochemistry of benzene induced by 193 and 253.7 nm excitations by matrix-isolation infrared spectroscopy using solid parahydrogen as a host matrix. In addition to the formation of benzvalene, fulvene, and Dewar benzene as photoproducts of the UV photolysis of benzene, we have confirmed the production of cyclohexadienyl radical in solid parahydrogen as an intermediate species for a ring-opening reaction to 1,3,5-hexatriene. Moreover, we have identified 1,3-hexadien-5-yne and o-benzyne as minor products of the photochemical reaction. The reaction mechanisms of these species based on the analysis of FTIR spectra and quantum chemical calculations are discussed.

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S 2 excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO 2 + H or CO + OH. On the other hand, no photoproducts were detected by S 1 excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

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High-resolution infrared spectra of β-alanine and its water clusters have been studied using solid para-H 2 FT-IR matrix-isolation spectroscopy. It is known that zwitterion forms of amino acids are more stable than neutral forms in water solutions and biological environments, but it is still under debate whether zwitterions are stable in small alaninewater clusters. We have investigated the stabilization effect of water molecules on the zwitterion form of β-alanine by codepositing H 2 O and β-alanine in solid para-H 2 . Through a comparison with theoretical calculations, as well as with crystalline β-alanine FT-IR spectra, the characteristic NH 3 N-H bending vibrational frequency for the zwitterionic form was identified. Analysis of the spectral peak temporal behavior shows that other proposed zwitterion peaks behave similarly to the characteristic NH 3 spectral peak. It has been shown that water can stabilize the zwitterionic form of gas phase amino acids, causing the zwitterion to form preferentially over the neutral form under certain conditions. The β-alanine zwitterion formation rate may be attributed to aggregation of small water clusters in the solid para-H 2 matrix. These findings provide insight into the behavior of amino acid zwitterion formation.