Spectroscopy and Photochemistry of Matrix-Isolated
Molecules

RESEARCH

METHOD

Matrix isolation

Spectroscopic investigations of molecules frozen in cryogenic solids are known since 1920s. The idea of using rare gas matrices to isolate and study unstable species was proposed independently in 1954, by G.C. Pimentel¹ and by G. Porter². This technique allowed investigation of: trapped free monomeric molecules, unstable radicals, tautomers, conformers, reaction intermediates and products, stable and unstable complexes, separated ions.

¹ E. Whittle, D. A. Dows, and G. C. Pimentel,
J. Chem. Phys. 22, 1943 (1954).

² I. Norman and G. Porter, Nature, Lond. 174, 508 (1954).

Scheme of the experiment

TOPICS

Research of the group concentrates mainly on investigations of unimolecular photochemical transformations of nucleic acid bases, their derivatives and analogues. Photochemical transformations of monomeric molecules of these chemical species, isolated in low-temperature matrices, are induced by excitation with UV or NIR light of an appropriate wavelength. Two inherent features of matrix isolation: [i] very low temperature, 10 K or lower, [ii] inert chemical environment, precluding interactions with any potential reagents, make this technique a very useful tool for traping and stabilizing primary products of photochemical transformations. Because the matrix cage (usually) does not allow migrations of reagents and photogenerated species, the photoreactions observed using matrix-isolation method concern, in the most part, photoisomerizations.

The photoinduced changes in the structure of investigated molecules are monitored by recording infrared (FTIR) spectra prior to UV irradiation as well as at consecutive stages of the photoproces. The products generated in a photoreaction are identified by comparison of their IR spectra with the spectra theoretically predicted for a series of guessed, trial structures. In most of the cases this procedure leads to unequivocal identification of the photoproduces species.

Several types of photoisomerization reactions were studied using this approach.

Photoinduced intramolecular proton transfer

– phototautomerism in carbonyl and thiocarbonyl heterocycles

(the first observation of this type of intramolecular proton transfer)

phototautomerism

J. Mol. Struct. 175, (1988), 91-96.
Spectrochim. Acta A 51, (1995), 1809–1826.

– phototautomerism in simple thioamides and selenoamides

Phys. Chem. Chem. Phys., 5, (2003) 1524-1529.
Chem. Phys. 298, (2004), 223–232.

PIDA mechanism:

J. Phys. Chem. A, 112, (2008) 13655–13661.

Proton tunneling (in darkness) following, in some cases, the phototautomeric reaction

J. Phys. Chem. A, 107 (2003), 6373–6380.

Photochemical syn-anti isomerization in N⁴-hydroxycytosines, N⁴-methoxycytosines, N²‑hydroxyisocytosines and in related compounds

UV irradiation

J. Phys. Chem. A, 110 (2006), 5038–5046.

UV-induced hydrogen-atom-transfer processes converting the amino-oxo tautomer of cytosine into the amino-hydroxy and imino-oxo forms.

CYTOSINE

Phys. Chem. Chem. Phys., 13 (2011) 9676-9684.

Photochemical ring-opening by cleavage of the bond in alpha position with respect to the carbonyl group

J. Am. Chem. Soc., 116 (1994), 1461–1467.
J. Phys. Chem. A, 107 (2003) 5913-5919.

Photochemical valence isomerization to the Dewar form

J. Am. Chem. Soc., 116 (1994), 1461–1467.

Photoisomerization reactions of monomeric thiophenol isolated in low-temperature argon matrices.

H-atom-transfer isomerization reactions dominate the unimolecular photochemistry of phenol and thiophenol confined in a solid argon matrix.

Phys. Chem. Chem. Phys., 17 (2015) 4888-4898.

Conformational changes induced by NIR irradiation of the molecules isolated in cryogenic matrices
NIR excitation of overtones of OH stretching vibrations led to rotation of the OH group in the cytosine molecule (in the hydroxy-form). Phys. Chem. Chem. Phys., 12 (2010) 9615-9618.	Excitation of overtone of OH stretching vibrations with tunable NIR laser light induced structural transformations of glycolic acid molecule. The higher-energy conformers were produced. J. Phys. Chem. A, 118 (2014), 5626–5635.
Long-range vibrational energy transfer from the initially excited OH group to the remote fragment which changes its orientation
NIR-induced transformations, converting one amino-thiol conformer of 2-thiocytosine into another by selective excitation to the overtone (or combination) of NH2 stretching vibrational states. The first observation of vibrational energy redistribution from the initially excited NH2 moiety to the remote SH group that changes its orientation J. Phys. Chem. A, 119 (2015), 9262–9271.
Excitations at wavenumbers corresponding to the overtone of the stretching vibration of the OH bond of the hydroxymethyl group as well as excitation of OH group directly attached to the heterocyclic ring led to conversion of one of the observed conformers into another. kojic acid J. Phys. Chem. A, DOI: 10.1021/acs.jpca.6b01275

Proton tunneling, the spontaneous processes changing the conformational structure
Cytosine, amino-hydroxy tautomer J. Chem. Phys. 136, (2012) 064511
Oxamic Acid J. Phys. Chem. A, 2015, 119, 2203−2210	2‑Furoic Acid J. Phys. Chem. A, (2015), 119, 1037−1047

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RESEARCH

METHOD

Matrix isolation

TOPICS

Photoinduced intramolecular proton transfer

Proton tunneling, the spontaneous processes changing the conformational structure