Hydrogen-atom tunneling in small thioamides: N-methylthiourea, thiobenzamide and 2-cyanothioacetamide.

The most stable thione tautomeric forms of N-methylthiourea, thiobenzamide and 2-cyanothioacetamide were isolated in low-temperature argon matrices. The higher-energy thiol tautomers of these compounds were generated upon irradiation of matrix-isolated monomers with UV (λ > 270 nm) light. For N-methylthiourea and thiobenzamide, kept in the dark at 3.5 K for a long period of time, a spontaneous thiol â†’ thione hydrogen atom tunneling transformation occurred. Only the thiol isomers with the favorably oriented hydrogen atom of the imino group underwent these hydrogen-atom tunneling processes. The other thiol isomers, with the hydrogen atom of the imino group oriented towards the sulfur atom, did not undergo the thiol â†’ thione conversion. For the photogenerated thiol forms of 2-cyanothioacetamide, no spontaneous thiol â†’ thione tautomeric transformation was detected. Instead, only the spontaneous conformational change of one S-H rotamer of the thiol 2-cyanothioacetamide tautomer into the other S-H rotamer was observed.

Photoinduced Long-Distance Hydrogen-Atom Transfer in Molecules with a 7-Hydroxyquinoline Frame and a Carbaldehyde or Aldoxime Group as the Intramolecular Hydrogen Transporting Crane.

The photochemical properties of monomeric 7-hydroxyquinoline substituted at position 8 with carbaldehyde or aldoxime groups were studied for the molecules isolated in solid Ar low-temperature matrices (at 10 K). It was experimentally demonstrated that upon UV excitation, both carbaldehyde and aldoxime groups act as intramolecular cranes transmitting hydrogen atoms from the hydroxyl group to the remote nitrogen atom of the quinoline ring. Furthermore, in the case of 7-hydroxyquinoline-8-aldoxime (and its derivatives), the second photochemical channel was activated upon UV (λ > 360 nm) excitation. This process involves syn-anti isomerization around the double C═N bond in the aldoxime group. The structures of the reactant hydroxy tautomeric form and the photoproduced isomers of the studied molecules were unequivocally determined by means of IR spectroscopy combined with theoretical predictions of the IR spectra of the candidate structures.

Effect of a Solid-Hydrogen Environment on UV-Induced Hydrogen-Atom Transfer in Matrix-Isolated Heterocyclic Thione Compounds.

To shed more light on the mechanisms of UV-induced hydrogen-atom-transfer processes in heterocyclic molecules, phototautomeric thione → thiol reactions were investigated for thione compounds isolated in low-temperature Ar as well as in n-H2 (normal hydrogen) matrices. These studies concerned thione compounds with a five-membered heterocyclic ring and thione compounds with a six-membered heterocyclic ring. The experimental investigation of 2-thioimidazole and 3-thio-1,2,4-triazole (thione compounds with a five-membered heterocyclic ring) revealed that for the compounds isolated in solid n-H2 only trace amounts of thiol photoproducts were photogenerated; even though for the same compounds isolated in the solid Ar matrix, the thione → thiol photoconversion was nearly total. In contrast to that, for 3-thiopyridazine and 2-thioquinoline (thione compounds with a six-membered heterocyclic ring) isolated in solid n-H2, the UV-induced thione → thiol conversion occurred with the yield reaching 25-50% of the yield of the analogous process observed for the same species isolated in solid Ar. The obtained experimental results allow us to conclude that the dissociation-association mechanism nearly exclusively governs the phototransformation in thione heterocycles with high barriers for tautomerization (such as thione compounds with a five-membered ring), whereas the strictly intramolecular hydrogen-atom shift contributes to the mechanism of hydrogen-atom transfer in thione heterocycles with lower barriers (such as thione compounds with a six-membered ring).

Photochemical Generation of Benzoazetinone by UV Excitation of Matrix-Isolated Precursors: Isatin or Isatoic Anhydride.

Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-temperature Ar matrixes. Upon UV (λ = 278 nm) excitation of isatin, monomers of the compound underwent decarbonylation and the remaining part of the molecule adopted the benzoazetinone structure or the structure of its open-ring isomer α-iminoketene. The same products (benzoazetinone and α-iminoketene) were generated by UV (λ = 278 nm) induced decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced α-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti isomers. Photoproducts generated upon λ = 278 nm irradiation of matrix-isolated isatin were subsequently exposed to λ = 532 nm light. That irradiation resulted in the shift of the α-iminoketene-benzoazetinone population ratio in favor of the latter closed-ring structure. The next irradiation at 305 nm caused the shift of the α-iminoketene-benzoazetinone population ratio in the opposite direction, that is, in favor of the open-ring isomer. Neither benzoazetinone nor its α-iminoketene open-ring isomer was generated upon UV (λ = 278 nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of this compound underwent such phototransformations as oxo → hydroxy phototautomerism or degradation of the five-membered ring with release of HNCO and CO. The efficiency of these photoconversions was low.

Conformational Isomerizations by Rotation around C-C or C-N Bonds: A Comparative Study on Matrix-Isolated Glycolamide and N-Hydroxyurea Excited with Near-IR Laser Light.

Conformers and near-IR-induced conformational transformations were studied for monomers of glycolamide isolated in low-temperature matrixes. Two conformational isomers of the compound, Tt and Cc, were trapped from the gas phase into solid Ar matrixes. Selective near-IR excitation of glycolamide molecules adopting the Tt form led to the Tt → Cc conformational conversion. Analogously, selective near-IR excitation of Cc conformers resulted in the Cc → Tt transformation. Monochromatic near-IR light, generated by frequency-tunable laser sources, was used for irradiation of matrix-isolated monomers. Near-IR-induced Tt → Cc and Cc → Tt conformational transformations occurred upon excitation of 2νOH, 2νaNH2, and 2νsNH2 overtones, as well as upon excitation of νaNH2 + νsNH2 combination modes. In spite of the structural similarity of glycolamide and N-hydroxyurea, no conformational conversions were observed for monomers of the latter compound isolated in Ar matrixes and excited with near-IR light. The comparison of the effects of near-IR excitations of glycolamide and N-hydroxyurea demonstrates that transformations involving rotation of molecular fragments around a single C-C bond occur much easier than transformations involving rotation of the fragments around a C-N bond. The efficiency of the latter conversions is extremely low.

Hydrogen-atom tunneling through a very high barrier; spontaneous thiol → thione conversion in thiourea isolated in low-temperature Ar, Ne, H2 and D2 matrices.

Spontaneous thiol → thione hydrogen-atom transfer has been investigated for molecules of thiourea trapped in Ar, Ne, normal-H2 (n-H2) and normal-D2 (n-D2) low-temperature matrices. The most stable thione isomer was the only form of the compound present in the matrices after their deposition. According to MP2/6-311++G(2d,p) calculations, the thiol tautomer should be higher in energy by 62.5 kJ mol-1. This less stable thiol form of the compound was photochemically generated in a thione → thiol process, occurring upon UV irradiation of the matrix. Subsequently, a very slow spontaneous conversion of the thiol tautomer into the thione form was observed for the molecules isolated in Ar, Ne, n-H2 and n-D2 matrices kept at 3.5 K and in the dark. Since the thiol → thione transformation in thiourea is a process involving the dissociation of a chemical bond, the barrier for this hydrogen-atom transfer is very high (104-181 kJ mol-1). Crossing such a high potential-energy barrier at a temperature as low as 3.5 K, is possible only by hydrogen-atom tunneling. The experimentally measured time constants of this tunneling process: 52 h (Ar), 76 h (Ne), 94 h (n-H2) and 94 h (n-D2), do not differ much from one another. Hence, the dependence of the tunneling rate on the matrix environment is not drastic. The progress of the thiol → thione conversion was also monitored for Ar matrices at different temperature: 3.5 K, 9 K and 15 K. For this temperature range, the experiments revealed no detectable temperature dependence of the rate of the tunneling process.

UV-Induced Hydrogen-Atom-Transfer Processes in 3-Thio-1,2,4-triazole Isolated in Ar and H2 Low-Temperature Matrixes.

The UV-induced thione → thiol phototautomeric reaction has been studied for monomeric 3-thio-1,2,4-triazole (3-ST) isolated in low-temperature Ar and n-H2 (normal hydrogen) matrixes. Prior to any UV irradiation, monomers of 3-ST isolated in solid Ar or solid n-H2 adopted mainly the most stable thione tautomeric form, as revealed by the IR spectra. Upon UV (λ > 275 nm) irradiation of 3-ST isolated in Ar matrixes, the IR bands due to this thione form decreased, while a set of initially weak bands increased in intensity. Growing bands indicated generation of a photoproduct, which was identified as the thiol tautomer with labile hydrogen atoms attached to sulfur and N(2) atoms. The UV-induced spectral changes allowed also identification of another minor thiol tautomer of 3-ST, which was present in the matrix prior to any irradiation and did not change its population upon exposure to UV light. The identification of the observed isomeric forms was supported by comparison of their separated experimental IR spectra with the spectra theoretically predicted for the various structures of 3-ST. The thione → thiol phototautomerization that was the main UV-induced process observed for 3-ST in Ar matrixes did not occur for monomers of the compound trapped in solid n-H2.

UV-induced hydrogen-atom transfer and hydrogen-atom detachment in monomeric 7-azaindole isolated in Ar and n-H2 matrices.

Photochemical transformations were investigated for monomers of 7-azaindole isolated in low-temperature Ar and normal-H2 (n-H2) matrices. The most stable N1H tautomer was the only form of the compound populated in Ar and n-H2 matrices before any irradiation. Upon exposure of Ar matrices to UV (λ > 270 nm) light, two higher-energy tautomers N7H and C3H were photoproduced. Additionally, spectral signatures of the photogenerated 7-azaindolyl radical were also found. All of these photoproducts were experimentally observed for the first time. So far, the N7H tautomer had been known only as a transient species, appearing upon relaxation of photoexcited hydrogen-bonded dimers or complexes. For 7-azaindole isolated in an n-H2 matrix and irradiated at λ > 270 nm, only the C3H tautomer and the 7-azaindolyl radical were photogenerated, whereas the N7H tautomer was not photoproduced at all. The drastic dependence of photogeneration of the N7H form on the matrix environment (solid Ar or solid n-H2) is as a characteristic feature of a specific class of UV-induced hydrogen-atom-transfer processes occurring in matrix-isolated heterocycles.

Conformers of Kojic Acid and Their Near-IR-Induced Conversions: Long-Range Intramolecular Vibrational Energy Transfer.

Conformational transformations were investigated for molecules of kojic acid trapped in low-temperature argon and nitrogen matrixes. Two conformers, differing from each other by 120° rotation of the hydroxymethyl (-CH2OH) moiety, were found to be populated in freshly deposited matrixes, prior to any irradiation. Matrixes containing isolated monomers of kojic acid were irradiated with narrowband, tunable near-infrared (near-IR) laser light. Excitations at wavenumbers corresponding to the overtone of the stretching vibration of the OH bond of the hydroxymethyl group led to conversion of one of the observed conformers into another. The direction of this conformational transformation depended on the wavenumber (within the 7126-7115 cm(-1) range) used for irradiation. The same conformational photoconversion was also observed to occur upon narrowband irradiation at much lower wavenumbers (from the 6468-6447 cm(-1) range). Near-IR light from this range selectively excites overtone vibrations of the OH group directly attached to the heterocyclic ring. Such an observation provides a convincing evidence of a long-range vibrational energy transfer from the initially excited OH group (directly attached to the ring) to the remote hydroxymethyl fragment which changes its orientation. Structural changes, occurring in matrix-isolated molecules of kojic acid upon near-IR excitation, were monitored by FTIR spectroscopy.

Intramolecular Vibrational Energy Redistribution in 2-Thiocytosine: SH Rotamerization Induced by Near-IR Selective Excitation of NH2 Stretching Overtone.

Near-IR-induced transformations, converting one amino-thiol conformer of 2-thiocytosine into another, were observed for monomers of the compound isolated in Ne, Ar, and N2 low-temperature matrixes. The two conformers involved in this phototransformation differ from each other by 180° rotation of the SH group. To induce the conversion, conformers of 2-thiocytosine were selectively excited to the overtone (or combination) NH2 stretching vibrational states, using very narrowband (fwhm <1 MHz) near-IR light generated in a tunable diode laser. The conformational changes were monitored by IR spectroscopy. The conformational transformation observed in the current work provides a clear evidence of the vibrational energy redistribution from the initially excited NH2 moiety to the remote SH group that changes its orientation.

Three conformers of 2-furoic acid: structure changes induced with near-IR laser light.

Conformers of 2-furoic acid were studied using the matrix-isolation technique combined with narrow-band near-IR excitations with tunable laser light. Two conformers of the compound were trapped from the gas phase into low-temperature Ar or Ne matrixes with the population ratio of nearly 1:1. The two forms differ from each other by 180° rotation of the carboxylic group with respect to the furan ring. In both structures, the OH group adopts the cis orientation, with its H atom directed toward the C═O bond of the O═C-O-H group. Narrow-band near-IR excitations of the OH stretching overtone vibrations resulted in transformation of one of the initially observed conformers into a third conformational structure. This near-IR-induced isomerization concerned rotation of the OH group from the initial cis orientation to the trans conformation with the hydrogen atom directed toward the oxygen atom of the furan ring. In the photoproduced conformer, the hydrogen-bond-like O-H···O interaction (between O-H and the oxygen atom of the furan ring) is rather weak. Nevertheless, this interaction stabilized the structure so that it was present in the matrix for several hours after the near-IR-induced generation. The spontaneous conversion of the photogenerated, higher-energy form back into the more stable conformer with the carboxylic group in cis orientation was monitored for 2-furoic acid isolated in Ar and Ne matrixes. The speed of this process was found to be dependent on temperature and on the matrix material. The experimentally determined half-life times of this conformational conversion occurring in the dark are t1/2 = 1390 min (Ar, 5.5 K); t1/2 = 630 min (Ar, 15 K); t1/2 = 240 min (Ne, 5.5 K). The three conformers of 2-furoic acid observed in the present work were identified by comparison of their infrared spectra with the spectra theoretically calculated for the candidate structures.

Tunable diode lasers as a tool for conformational control: the case of matrix-isolated oxamic acid.

A tunable diode laser was applied as a source of narrowband near-infrared light used to manipulate the structure of the molecule of oxamic acid. Monomers of the most stable conformer I of the molecule, with the trans orientation of the O═COH group and the trans orientation of the O═CC═O fragment, were trapped from the gas phase in low-temperature argon, neon, and nitrogen matrixes. Monomers of oxamic acid, isolated in argon or neon matrixes, were then irradiated with narrowband near-IR light from the diode laser tuned at 6833 (Ar) or 6840 cm(-1) (Ne). Upon such irradiation another conformer, II, of oxamic acid was generated, with cis orientation of the O═COH group and trans orientation of the O═CC═O fragment. Both forms were identified by comparison of their experimental mid-IR spectra with the spectra theoretically calculated for I and II. Subsequent irradiation of the matrix at 6940 (Ar) or 6991 cm(-1) (Ne), where absorption appeared in the near-IR spectrum of the photoproduct, led to photoconversion of conformer II into form I. In a series of subsequent irradiations at 6833(Ar)/6840(Ne) cm(-1) and at 6940(Ar)/6991(Ne) cm(-1), the population of oxamic acid molecules was selectively shifted several times from I to II and vice versa. As far as we know, this is the first reported study where a tunable diode laser source of narrowband near-IR light was used to manipulate the structure of a molecule. Spontaneous II → I transformation was observed for Ne and Ar matrixes kept in the dark and at cryogenic temperature.

Near-infrared laser-induced generation of three rare conformers of glycolic acid.

Structural transformations were induced in conformers of glycolic acid by selective excitation with monochromatic tunable near-infrared laser light. For the compound isolated in Ar matrixes, near-IR excitation led to generation of two higher-energy conformers (GAC; AAT) differing from the most stable SSC form by 180° rotation around the C-C bond. A detailed investigation of this transformation revealed that one conformer (GAC) is produced directly from the near-IR-excited most stable conformer. The other higher-energy conformer (AAT) was effectively generated only upon excitation of the primary photoproduct (GAC) with another near-IR photon. Once these higher-energy conformers of glycolic acid were generated in an Ar matrix, they could be subsequently transformed into one another upon selective near-IR excitations. Interestingly, no repopulation of the initial most stable SSC conformer occurred upon near-IR excitation of the higher-energy forms of the compound isolated in solid Ar. A dramatically different picture of near-IR-induced conformational transformations was observed for glycolic acid isolated in N2 matrixes. In this case, upon near-IR excitation, the most stable SSC form converted solely into a new conformer (SST), where the acid OH group is rotated by 180°. This conformational transformation was found to be photoreversible. Moreover, SST conformer, photoproduced in the N2 matrix, spontaneously converted to the most stable SSC form of glycolic acid, when the matrix was kept at cryogenic temperature and in the dark.

Near-IR-induced, UV-induced, and spontaneous isomerizations in 5-methylcytosine and 5-fluorocytosine.

Monomeric 5-methylcytosine (5mCyt) and 5-fluorocytosine (5FCyt) were studied using the matrix-isolation method. In 5mCyt and 5FCyt, the most stable form, dominating in low-temperature matrixes, is the amino-hydroxy (AH) tautomer. For both compounds, irradiation of the matrixes with near-IR laser light or with broadband near-IR or mid-IR light induces interconversions between the two rotamers of tautomer AH. In addition, for matrixes kept in darkness, a spontaneous tunneling conversion of the higher-energy hydroxy conformer (with the OH group directed toward the N3 atom) into the lower-energy form (OH directed toward N1) was occurring, with half-life time of 70 min for 5mCyt and 127 min for 5FCyt. These tunneling processes are much faster than that found for unsubstituted cytosine, where the half-life time is more than 30 h. UV irradiation of 5mCyt (at 316 nm) led to phototautomeric conversion of the amino-oxo form into the amino-hydroxy tautomer. Another phototransformation induced by irradiation of 5mCyt at 316 nm was the cleavage of the C-N bond in the amino-oxo form, resulting in generation of the open-ring conjugated isocyanate product. Irradiation of 5mCyt at shorter waves (λ ≤ 310 nm) induced the syn-anti photoisomerization within the imino-oxo forms of the compound. For matrix-isolated 5FCyt, the amount of the amino-oxo form was very small (with respect to the amino-hydroxy tautomer), while the imino-oxo isomers were not detected at all.

Conformational transformation in squaric acid induced by near-IR laser light.

Two conformers of monomeric squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) were studied using the matrix-isolation method. Both forms of the compound, differing in rotation of one of the OH groups by 180°, were trapped from the gas phase into a low-temperature nitrogen matrix, whereas only the lowest-energy conformer was trapped in solid argon and in solid neon. Narrowband near-infrared laser light was used to induce transformation of the most stable form of squaric acid (having C2v symmetry) into the higher-energy conformer (Cs symmetry). Effective stabilization of the photogenerated species occurred only for the compound isolated in a nitrogen matrix. Moreover, the stabilization of the higher-energy Cs conformer of squaric acid by the solid nitrogen environment was found to strongly depend on the trapping site in the matrix. The spectroscopic characterization of the higher energy Cs conformer is reported here for the first time.

Photochemical isomerizations of thiosemicarbazide, a matrix isolation study.

Two thione conformers of monomeric thiosemicarbazide were trapped from the gas phase into a low-temperature Ar matrix. A phototransformation converting the less stable form of the compound into the most stable conformer was induced by irradiation with near-IR (λ = 1462 nm) or UV (λ > 320 nm) light. This photoeffect allowed separation of the IR spectra of the observed thione forms. The structures of both observed isomers were identified by comparison of the separated experimental IR spectra with the spectra theoretically predicted for two most stable forms of the compound. The population ratio of the two conformers in an Ar matrix, prior to any irradiation, was estimated to be equal ≈2:1. Irradiation of matrix-isolated thiosemicarbazide with shorter-wavelength UV (λ > 270 nm) light induced a phototautomeric reaction generating thiol forms of the compound.

UV-induced hydrogen-atom transfer in 3,6-dithiopyridazine and in model compounds 2-thiopyridine and 3-thiopyridazine.

Monomeric 3,6-dithiopyridazine (3-mercapto- 6(1H)-pyridazinethione) was studied using the matrix-isolation method combined with quantum chemical calculations. The monomers of 3,6-dithiopyridazine, trapped from the gas phase into a low-temperature Ar matrix, were found to adopt the thione-thiol structure. In agreement with this experimental observation, the thione-thiol form was predicted (at the QCISD level) to be more stable by 13.5 kJ mol(-1) and by 39.6 kJ mol(-1) than the dithiol and the dithione tautomers, respectively. Monomers of 3,6-dithiopyridazine isolated in Ar matrixes were then irradiated with broadband UV (λ > 335 nm) light. Upon such irradiation, the thione-thiol form of the compound converted into the dithiol tautomer. The same phototransformation was observed when monochromatic λ = 385 nm laser light was used for irradiation. This allowed a first observation and spectral characterization of the dithiol form of 3,6-dithiopyridazine. Subsequent irradiation of the UV-generated dithiol tautomer with shorter-wavelength UV (λ > 275 nm) light led to partial repopulation of the thione-thiol form. Spectral signatures of the analogous photoreversibility were also found for the phototautomeric transformation in the model compound 3-thiopyridazine. The reliability of the QCISD predictions of relative energies of thiol and thione tautomeric forms was tested on the archetype example of 2-thiopyridine. For this compound, the comparison of the computed relative energy 10.9 kJ mol(-1) with the experimental estimate 10.0 ± 1.5 kJ mol(-1) (both in favor of the thiol form) was more than satisfactory.

NIR-laser-induced selective rotamerization of hydroxy conformers of cytosine.

The relative populations of two amino-hydroxy conformers of cytosine, differing in rotation of the OH group by approximately 180 degrees , were selectively and repeatedly manipulated with narrowband, near-infrared laser light. For cytosine monomers isolated in a low-temperature Ar matrix, laser irradiations at 7013 cm(-1) and at 7034 cm(-1) were found to induce effective transformations of the two conformers into each other.

Positive identification of UV-generated, non-hydrogen-bonded isomers of o-hydroxybenzaldehyde and o-hydroxyacetophenone.

Non-hydrogen-bonded isomers were photogenerated by UV (lambda > 335 nm) irradiation of o-hydroxybenzaldehyde (salicylaldehyde) and o-hydroxyacetophenone monomers isolated in low-temperature Ar matrixes. These photoisomerizations were found to be photoreversible. Upon shorter wavelength (lambda > 235 nm or lambda > 270 nm) UV irradiation, the initial forms of the compounds (with intramolecular hydrogen bonds) were partially repopulated. The structures of the photogenerated non-hydrogen-bonded isomers of both compounds were positively identified by comparison of their IR spectra with the spectra theoretically calculated [at the DFT(B3LYP)/6-311++G(2d,p) level] for all possible non-hydrogen-bonded isomers of the studied compounds. The experimental IR spectra of the photoproducts generated from o-hydroxybenzaldehyde and o-hydroxyacetophenone are very well reproduced only by the theoretical spectra predicted for the isomers with both OH and formyl (or acetyl) groups rotated by 180 degrees, with respect to the initial, most stable hydrogen-bonded conformer. Excellent agreement between experiment and theoretical prediction provides a basis for a very reliable identification of the photoproduced isomers of o-hydroxybenzaldehyde and o-hydroxyacetophenone.

UV-induced oxo --> hydroxy unimolecular proton-transfer reactions in hypoxanthine.

Monomers of hypoxanthine isolated in low-temperature Ar matrixes were studied using Fourier transform infrared spectroscopy. Two most stable tautomeric forms of hypoxanthine: oxo-N(9)-H and oxo-N(7)-H as well as a very small amount of the minor hydroxy-N(9)-H tautomer were observed in Ar matrixes directly after their deposition. UV irradiation of the matrixes induced conversion of the oxo-N(9)-H and oxo-N(7)-H tautomers of the compound into the hydroxy-N(9)-H and hydroxy-N(7)-H forms, respectively. Upon exposure of the matrixes to the UV (lambda > 270 nm) light, the oxo-N(9)-H --> hydroxy-N(9)-H phototautomeric reaction dominated strongly over the oxo-N(7)-H --> hydroxy-N(7)-H phototransformation. The latter phototautomeric reaction occurred effectively when matrix-isolated hypoxanthine was irradiated with shorter-wavelength (lambda > 230 nm) UV light. Thanks to this wavelength dependency, it was possible to clearly distinguish the oxo --> hydroxy photoreaction within the N(9)-H tautomers from the analogous phototautomeric process within the N(7)-H tautomers. All of the observed isomers of hypoxanthine (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-31++G(d,p) level of theory.