Scientific paper Photoinduced Tautomerism of 2-Thiobarbituric Acid Studied by Theoretical and Experimental Methods Rumyana I. Bakalska and Vassil B. Delchev* Faculty of Chemistry, University of Plovdiv, BG-4000 Plovdiv, Tzar Assen 24 Str., Bulgaria * Corresponding author: E-mail: vdelchev@uni-plovdiv.bg Received: 10-03-2011 Abstract Combined, theoretical and experimental, investigation was performed to study the mechanism of the photoinduced tautomerism of 2-thiobarbituric acid (TBA). The irradiation of the solution of TBA in polar aprotic solvent with UV light (maximum at 366 nm) showed oxo-hydroxy photoisomerization of the triketo form of TBA to the hydroxy-imino tau-tomer. The studied mechanisms (TD DFT) of the photoinduced NH and OH dissociations in the keto and enol tautomer revealed that the proton detachment in the triketo tautomer occurs in the bright ^Sa* excited state. In the hydroxy-imi-no tautomer this mechanism is driven by the repulsive *na* excited state. The excited-state relaxation mechanisms occur by low-lying So-Sj conical intersections. Keywords: Conical intersections, photochemistry, proton detachment, TD DFT calculations, 2-thiobarbituric acid, UV irradiation 1. Introduction Thiobarbituric acid (TBA) is a sulfur derivative of barbituric acid in which the oxygen atom at the second position is replaced by a sulfur atom. Being analogue of barbituric acid, TBA shows significant difference in the enolization tendency as well as in the spectral features.1 The interest in barbiturates is provoked by the fact that they were in therapeutic use as CNS depressants (sedation, hypnotic, preanaesthetic, anticonvulsant activity, headache products etc.) for a long time.2 Barbiturates are strongly electron-withdrawing because they gain aromatic stabilization upon reduction.3 This property has been exploited in the preparation of molecules which possess pronounced quadratic non-linear optical properties - of interest for potential applications in optoelectronic and photonic technologies.4,5 TBA is an object of a large number of investigations. As concerns the spectroscopic features of this compound, Mendez et al have identified one intense UV band of this compound in methanol solution at 283 nm with a well-defined shoulder at 265 nm.6 They have discussed also the nature of the two less intensive absorption bands at 244 and 227 nm. Further, the analysis of the vibration spectrum of TBA in the solid state has demonstrated that the compound exists in two tautomeric forms in equal amounts: the triketo tautomer of TBA and one enol tautomer.6,7 This is the enol tautomer with a CH group at the fifth position. The same enol form of TBA has been found by Goel in the solid state of TBA.8 In solvents with a large dielectric constant the enol tautomer predominates.9 Zuccarello et al have performed a detailed study of the tautomeric equilibria in TBA.1 They have discussed the presence of neutral tautomers as well as their anions and protonated forms in water solution at different pH. However, no comment has been done for the potential possibility of TBA to phototautomerize upon UV-irradiation. Millefiori et al have studied theoretically (at the se-miempirical level AM1) the possible tautomers of thiobar-bituric acid.10 They have found, in agreement with the experiment, that the barbiturate ring is essentially planar.10 The investigation of Chierotti et al reports about the possibility of the enol and keto tautomers of TBA to participate in intermolecular H-bonding and thus to form large clu-sters.7 Both forms are stable and form polymorphous crystals at different conditions. Martos-Calvente et al have discussed the usefulness and accuracy of the DFT theory for description of the tautomeric equilibria thienol-thione.11 They have found that the calculated vibration spectra of the model compounds fit fairly well with the experimental IR spectra. Moreover, DFT calculations interpret rather accurately the SH stretching vibration.11 The purpose of the current research is to throw light upon the mechanism of photoinduced tautomerism of TBA. Several theoretical studies have revealed that in the most cases (e.g. pyrimidine derivatives) this mechanism is connected to the dissociation of the proton and subsequent association to another atom (O, C etc.) by the so-called PIDA mechanism (photoinduced dissociation-association).12-14 It is believed (for pyrimidine bases) that this mechanism is driven by the repulsive 1no* excited state.12,13 2. Computational Methods All tautomeric forms of TBA were optimized at the TD DFT (B3LYP) and CC2 theoretical levels (^-resulted geometries are given in ESI). Subsequent frequency calculations were performed to prove that the molecules are located in real minima. The B3LYP-optimized geometries of the ground states of the tautomers were used for subsequent calculations of the vertical excitation energies of the singlet electronic states. The obtained vertical excitation energies were compared to the experimental UV bands (recorded in ace-tonitrile) of non-irradiated and irradiated samples of TBA. All calculations were performed with the aug-cc-p-VDZ basis functions and the program packages GAUSSIAN 03, and TURBOMOLE (for the CC2-calculations given in ESI) with CS symmetry restrictions.15-19 3. Experimental Methods 2-Thiobarbituric acid (anhydrous) was purchased from Fluka and acetonitrile was from Lab-Scan (super gradient). Photochemical irradiations were carried out in a standard Pyrex immersion reaction vessel. As a UV-light source we used a high pressure mercury lamp TQ 150 with Quartz filter (Heraeus Noblelight, Germany). The solution (0.0606 g of 2-thiobarbituric acid in 250 mL acetonitrile) was deoxygenated with nitrogen (30 min). Nitrogen was bubbled trough the solution during the irradiation (1 h). The course of the reaction was followed by UV-spectroscopy at every 10 min. The UV spectra were recorded with a Cintra 101 UV/VIS spectrophotometer. The IR-spectra of irradiated and non-irradiated samples were recorded with VERTEX 70 FT-IR spectrometer (Bruker Optics) as KBr discs. For the preparation of the KBr disc, acetonitrile of the part of irradiated sample (10 mL) was insufflated with nitrogen under vacuum in a closed vessel without heating. The contact of the irradiated solution with oxygen (air) caused oxi-reduction process and elementary sulfur is produced quickly. 4. Results and Discussion 4. 1. Ground-State Equilibrium Geometries of the Tautomers The ground-state equilibrium geometries of several tautomeric forms of TBA were optimized at the B3LYP/ aug-cc-pVDZ level. They are illustrated in Fig. 1 and several structural parameters are listed in Table 1. We chose the tautomers (E, E1, E2) that can be formed directly from the triketo form and two extra tauto-mers which can be obtained by tautomer E1 (E3 and E4). In this way, we comprised all kind of proton transfer mechanisms in TBA: and C-H-O, S-H-N, O-H-N. Moreover, the IR spectra of irradiated TBA were informative enough to help decide which tautomers to take into consideration in the present study. Figure 1. Optimized structures of the tautomeric forms of 2-thio-barbituric acid. In agreement with previous investigations,7,10 the data in Table 1 show that all tautomers are planar. In the tautomers K, E, E1 and E3 the CH2 hydrogens are symmetrically located with respect to the molecular plane. The presence of a symmetry plane facilitates the assignment of the molecular orbitals and the electronic excited states by symmetry. By the symmetry rules, we classified the molecular orbitals in two groups: with symmetry a' - no / nN / o-MO; and with symmetry a'' - n-MO. Therefore, the electronic excited states were divided into two Table 1. Selected structural parameters of the tautomers of TBA Paramater K E E1 E2 E3 E4 r^C,)* 1.378 1.387 1.290 1.380 1.298 1.284 r(C2N3) 1.379 1.395 1.383 1.366 1.399 1.380 r(N3C4) 1.394 1.383 1.397 1.420 1.289 1.376 r(C4C5) 1.514 1.515 1.510 1.449 1.491 1.355 r(C5C6) 1.514 1.497 1.532 1.361 1.534 1.465 r(NA) 1.394 1.287 1.404 1.373 1.396 1.424 KC4O9) 1.215 1.217 1.213 1.222 1.335 1.354 r(C6O8) 1.214 1.338 1.214 1.348 1.215 1.222 r(CA) 1.660 1.656 1.778 1.669 1.766 1.789 6(N1C2N3C4)** 0.0 0.0 0.0 0.0 0.0 0.0 6(C2N3C4C5) 0.0 0.0 0.0 0.0 0.0 0.0 O^C^) 0.0 0.0 0.0 0.0 0.0 0.0 O^C^) 0.0 0.0 0.0 0.0 0.0 0.0 GCC^C,) 0.0 0.0 0.0 0.0 0.0 0.0 6(H12C5C4N3) 122.9 122.5 123.1 180.0 122.1 180.0 - 0.0 - - - - - - - 180.0 - - 6(H10S7C2N1) - - 0.0 - 0.0 0.0 0(HnO9C4N3) - - - - 0.0 - 6(HBO9C4N3) - - - - - 180.0 * bond lengths in A; ** dihedral angles in deg. groups as well: with symmetry A' - / 1oo* / :no*; with symmetry A'' - 1nn* / 1on* / 1no* etc. 4. 2. Vertical Excitation Energies and Experimental UV Maxima The experimental UV spectrum of non-irradiated solution (in acetonitrile) of TBA shows broad bands at 280 and 228 nm. These bands agree with the UV absorption maxima of TBA in methanol which have been found by Mendez et al at 283 nm and 227 nm.6 In order to understand the nature of the electron transitions responsible for these UV-absorption maxima we calculated, at the B3LYP level, the vertical excitation energies of the six tautomers (Fig. 1). The results are collected in Table 2. a) b) 0.65 0.55 0.45 0.35 0.25 0.15 0.05 irradiated ii cm-irradiated 1800 1600 1400 wavenumbers, cm 1200 1000 -1 Figure 2. Selected areas (a) 3750-2750 cm '; (b) 1900-1000 cm 1 of the experimental IR spectra of non-irradiated (blue) and irradiated (red) TBA. The spectra were recorded in KBr discs. Table 2. Calculated and experimental (UV, in acetonitrile) vertical excitation energies of the tautomeric forms of thiobarbituric acid K E B3LYP Exp. B3LYP Exp. eV nm eV nm eV nm eV nm nSn* (A") 3.142 395 1nSn* (A") 2.539 488 non* (A") 4.451 279 1nn* (A') 4.373 284 4.055 306 nn* (A') 4.641 267 4.432 280 V* (A") 4.409 281 no^* (A") 4.882 254 ^n* (A") 4.697 264 nSn* (A") 5.246 236 1nSn* (A") 4.805 258 nn* (A') 5.454 227 5.443 228 ^o* (A') 5.110 243 5.065 245 nSo* (A') 5.513 225 1nn* (A') 5.142 241 nn* (A') 5.750 216 1nn* (A') 5.581 222 nSn* (A") 5.783 214 1nSn* (A") 5.686 218 no* (A") 5.913 209 ^o* (A') 5.714 217 5.881 211 E1 E2 non* (A") 3.713 334 1nSn* (A") 3.657 339 on* (A") 4.568 271 1nSn* (A") 4.406 281 nn* (A') 4.930 251 1nn* (A') 4.459 278 4.432 280 nNn* (A") 4.990 248 1nn* (A') 4.788 259 nn* (A') 5.414 229 1no* (A") 4.898 253 no* (A") 5.543 224 1nSo* (A') 4.928 252 noO* (A') 5.622 221 1non* (A") 5.150 241 non* (A") 5.689 218 1nn* (A') 5.272 235 5.443 228 noO* (A') 6.233 199 1non* (A") 5.348 231 no* (A") 6.263 198 1nSo* (A') 5.494 226 E3 E4 non* (A") 3.430 361 1non* (A") 4.417 281 n^n* (A") 4.159 298 1non* (A") 4.816 257 nn* (A') 4.378 283 1nn* (A') 5.020 247 nn* (A') 5.084 244 1no* (A") 5.052 245 %n* (A") 5.336 232 1noo* (A') 5.176 240 non* (A") 5.459 227 1no* (A") 5.198 239 no* (A") 5.930 209 1no* (A") 5.346 232 noo* (A') 5.931 209 1noo* (A') 5.355 232 no* (A") 6.150 202 1nn* (A') 5.531 224 nn* (A') 6.212 200 ^n* (A") 5.562 223 Comparing the theoretical and experimental excitation energies one can say that the non-irradiated solution contains tautomer K and E2, in agreement with Ref. 6. The aforementioned bands correspond to n ^ n* transitions in these tautomers. In fact the nS ^ o* electron transition of the tautomer K is also included in the broad band at 228 nm (5.443 eV). It contributes to the broadening of the experimental band (the nSo* excited state is a bright state - spectroscopically active sate). The molecular orbitals involved in the electronic transitions are illustrated in the supplementary information (ESI). The calculated vertical excitation energies of the low-lying dark :nSn*excited states are rather small. The reason might be the large diffuseness of the electron density of the sulfur atom as compared to the oxygen atom. The dark :nSn*excited states of all SH-tautomers (E1, E3, E4) have higher vertical energies and the low-lying dark state is the 1nOn* excited state. In the experimental UV spectra of the non-irradiated and irradiated samples of TBA we did not find any bands corresponding to the dark states which were calculated. 4. 3. Vibration Analysis of the Reactant and the Photoproducts The experimental IR spectrum of TBA (KBr disc) has been reported by Mendez.6 Unfortunately, the spectrum is rather structureless and featureless at the large and medium wavenumbers. Nevertheless, it is fairly informative with respect to the bands of the tautomers which Mendez has described.6 The band assignment has shown that TBA exists in two tautomeric forms - tautomer K and tautomer E2. We registered the IR spectra of the irradiated and non-irradiated TBA, whose selected bands are compared in Fig. 2. The presented cuts comprise the most informative bands distinguishing the tautomers of TBA. Their frequencies (theoretical and experimental) are listed in Table 3. The theoretically calculated frequencies were used to assign accurately the experimental bands which are fairly identical with those reported by Mendez for solid TBA.6 For non-irradiated TBA, the most intensive band at Table 3. Calculated and experimental (KBr disc) characteristic stretching vibrations of tautomeric forms of thiobarbituric acid K E Assign. Theoretical Exp. Assign. Theoretical Exp. non scaled scaled non scaled scaled vsNH 3580 / 32** /120*** 3354 3234 vOH 3708 / 85 / 133 3469 3346-3666 v> 3576 / 8 / 107 - - vNH 3571 / 54 / 68 3346 3192-3086 v>2 3104/ 1 /72 2926 3109 vasCH2 3094 / 0 / 98 2917 2928 vCH9 s 2 3068 / 6/2 2894 2876 vCH, s2 3059/0/276 2886 2862 vsC=O 1792 / 364 / 83 1747 1722 vC=O 1777 / 551 / 45 1733 1708 vasc=o 1772 / 603 / 41 1729 1682 vC=N 1671 / 634 / 157 1638 1686 vC=S 1539 / 560 / 50 1519 1566 vC=S 1473 / 233 / 73 1460 1531 vasNCN 1429 / 207 / 13 1421 1398 vC-O 1444 / 149 / 1 1434 1381 vC=S 1143 / 179 / 28 1163 1165 vC=S 1150 / 357 / 67 1170 1156 E1 E2 vNH 3575 / 60 / 80 3349 vOH 3807 / 122 / 122 3558 3418-3564 vasCH2 3104 / 1 /63 2926 vsNH 3606 / 89 / 66 3377 3234 vsCH2 3066 / 1 / 149 2892 vasNH 3590 / 78 41 3363 - vSH 2674 / 11 / 108 2540 vCH 3237/4/100 3046 2876 vsC=O 1795 / 321 / 52 1749 vC=O 1756 / 695 / 59 1714 1699 vasC=O 1767 / 317 / 47 1724 vC=C 1681 / 376 / 43 1647 1616 vC=N 1608 / 823 / 55 1581 vC-O 1266 / 104 / 1 1274 - vC-N 1217 / 82 / 14 1230 vC-N,C-O 1230 / 177 / 12 1242 1244 vC-S 995 / 63 / 1 1030 vC=S 1145 / 202 / 7 1165 1165 E3 E4 vOH 3708 / 79 / 155 3469 vOH 3807 / 106 / 122 3558 vasCH2 3095 / 0 / 79 2918 vNH 3610 / 93 / 74 3381 vsCH2 3058/ 1 / 192 2885 vCH 3216/3/119 3027 vSH 2663/3 / 117 2530 vSH 2678 / 10 / 91 2543 vC=O 1763 / 327 / 68 1721 vC=O 1734 / 343 / 52 1695 vC=N 1677 / 648 / 23 1643 vC=C 1707 / 657 / 38 1670 vC=N 1532 / 654 / 104 1513 vC=N 1607 / 412 / 9 1581 vC-O 1450 / 100 / 9 1439 vC-O 1270 / 133 / 5 1278 vC-S 1116 / 285 / 16 1139 vC-S 1140 / 11 /3 1161 * The scaled frequencies were found by the equation vexp = 8.899vtheor + 136.2. ** IR intensity / *** Raman activity 3109 cm-1 in Fig. 2a was assigned to the CH2 asymmetric stretching vibration. The low intensity and shallow band in the interval 3346-3666 cm-1 was referred to the OH stretching vibration in tautomer E2. The band at 3234 cm-1 was assigned to the symmetric NH stretchings in the tautomeric forms K and E2. The bands at 2926, 2894, and 2876 cm-1 were referred to the vas and vs CH2 stretching vibrations in tautomer K, and the CH stretch of the tautomer E2. The characteristic C=O stretching vibrations of the tautomers K and E2 (Fig. 2b, Table 3) were recorded at 1722 (K, vs), 1682 (K, vas), and 1699 cm-1 (E2). The C=S stretchings in the two forms were registered under 1600 cm-1: the characteristic band at 1566 cm-1, which is an indication that the thione form prevails in the solid state. The intensive band at 1165 cm-1 is also a proof in this aspect. The comparisons of the theoretical and experimental stretching vibrations of the tautomer K led to the following correlation equation, v = 8.899vlheor + 136.2, with a correlation coefficient of 0.989. This equation was used to scale the theoretical frequencies of all enol and thienol tautomers (see Table 3). The value of the correlation coefficient confirms the high accuracy of the B3LYP method for the prediction of the harmonic vibration frequencies of organic compounds. The irradiated TBA shows a broad band in the interval 3192-3086 cm-1 which was assigned to the NH stretching vibration of the tautomer E. This band has a clear shoulder in the interval 3346-3666 cm-1 which comes from the OH stretching of the same tautomer. The broadening of the OH and NH bands indicates that the enol tautomer E forms H-bonds, inter- or intramolecular. Irradiated TBA shows bands at 2928 and 2862 cm-1 assigned to the asymmetric and symmetric CH2 stretching vibrations. This is an indication that the CH2 group was not affected by the UV-irradiation. This finding excludes tautomers E2 and E4 as photoproducts in the final photomixture. The calculations showed that all thienol tautomers should have a band for the S-H stretching vibration at about 2500 cm-1. In the experimental IR spectra of irradiated and non- irradiated TBA such characteristic band was not registered. The IR spectrum of the irradiated TBA comprising the interval 1900-1000 cm-1 is presented in Fig. 2b. The bands at 1708 and 1686 cm-1 were assigned to the C=O and C=N stretching vibrations of the tautomer E. The C=S bands were recorded at 1531 and 1156 cm-1, respectively. The band at 1381 cm-1 assigned to the C-O single-bond stretching vibration is an indication for the presence of an OH group in the photoproduct. The band at 1273 cm-1 (in the non-irradiated and irradiated spectra) was assigned, in agreement to Mendez,6 to the CH in plane bending vibration in tautomer K. In the irradiated TBA this band has low intensity, which means that tautomer K dips out in the course of the UV-irradiation. The analysis of the vibration spectra of the two samples (irradiated and non-irradiated) showed that the main photoproduct of the TBA should be the tautomer E. In other words, the UV-irradiation of TBA leads to the proton migration from the N-atom to the O-atom of one of the carbonyl groups. The second carbonyl group and N-H bond of TBA remain non-affected by the UV-light. 4. 4. Excited-State Reaction Paths In order to explain the PIDA mechanism in TBA we studied the excited-state reaction paths of the NH dissociation in the tautomer K and the OH dissociation in the tautomer E. We believe that these processes are directly involved in the photoinduced keto-enol tautomerism of TBA. The excited state reaction paths are illustrated in Fig. 3. In both cases, the relative energies (£rel) were determined by the energies of the ground-state equilibrium geometries of the tautomers K and E, respectively. Fig. 3a illustrates the excited-state reaction paths of the H-detachment process in the tautomer K. The energy path of the ground state clearly shows the minimum of the ground state found at the N1-H10 distance of 1.015 A. The excited-state reaction paths of the low-lying 1nn* excited state and the second 1nn* excited state show crossings with the excited-state reaction paths of the bright 1nSo* and the repulsive 1no* excited state via conical intersections. In other words, the proton detachment process is driven by the bright 1nSo* excited state than by the repulsive 1no* excited state. Furthermore, the second 1nn* excited state plays much more pronounced role for the NH dissociation than the low-lying 1nn* excited state. The reason is that the system delivery from the second 1nn* excited state to the bright 1nSo* excited state passes through much lower energy barrier than the low-lying 1nn* excited state. The excited-state reaction path of the 1nSo* excited state leads to a low-lying conical intersection So-S1 which mediate the radiationless relaxation of the excited system. The population of the low-lying 1nSn* and 1nOn* excited states, in the Franck-Condon region of the tautomer K, can be achieved via internal conversions through conical intersections with the excited-state reaction paths of the 1nSo* excited state. This population would lead to weak bands in the experimental fluorescent spectrum of TBA. b) > 4 a; 1U 3 w ° ° P fioT* 9 0 n a a ifpS„o w 3 9 8 o o o ° O 8 9 O 8 ° ° ° ° s ^Jg o / 1n0ji* o 0 P O o w 12 \j"'0 13 V4 3 r / \ 0 9 1.4 1.9 2.4 Nt—H10 (Angstrom) Figure 3. Excited-state reaction paths of the (a) NH dissociation process of the tautomer K, and (b) OH dissociation process of the tautomer E. The excited-state reaction paths of the 1nSo* and 1no* excited states show minima at about N1-H10 = 1.666 À. In Fig. 3b are depicted the excited-state reaction paths for the OH dissociation of the tautomer E. These curves are similar to the energy profiles shown in Fig. 3a. However, there is one significant exception: the excited-state reaction path of the 1nSo* excited state does not lead to a low-lying So-S1 conical intersection (or a crossing with the ground-state reaction path). A crossing exists between the reaction paths of the 1no* excited state and the So state. It defines a low-lying conical intersection of the type So-S1. Such crossing is usually observed in pyrimidi-ne bases.14 Since the repulsive 1no* excited state is a dark state, the OH dissociation of the tautomer E would occur not as easy as the NH dissociation of the tautomer K. Furthermore, the tautomer K was found to be the most stable form of TBA which means that the irradiation of TBA with UV light would lead to the accumulation of the tautomer E in the final photomixture. The excited-state reaction paths of the 1no* and 1nSo* excited states show minima at the N1-H10 distance of 1.624 À. The minima are deeper than those of the NH dissociation of the tautomer K. Comparing the ground state minima in Fig. 3 one can see that the minimum of the tautomer E is much deeper than the minimum of the tautomer K. It means, that the second order Jahn-Teller effect should be observed in a greater extent in the tautomer K than in the tautomer E. 5. Conclusion We performed a combined, theoretical and experimental, study of the photoinduced tautomerism of TBA. In agreement with previous investigations,6,10 our study showed that the triketo tautomer of TBA is predominant in the solid state. The UV-irradiation of TBA leads to the accumulation of the hydroxy-imino tautomer E as the major photoproduct. The theoretical calculations performed at the TD DFT level showed that the tautomer K transforms photochemically into tautomer E via low-lying So-S1 conical intersection(s). This process predominantly occurs in the bright 1nSo* excited state. This state of the tautomer K can be populated from the second 1nn* excited state through internal conversion and an appropriate conical intersection. The photochemical OH dissociation of the tau-tomer E is driven by the repulsive 1no* excited state. 6. Acknowledgements We thank Mrs. Neda Danova, Dr. Plamen Penchev, and Dr. Maria Stoyanova from the Chemical faculty of the University of Plovdiv for the recording of the UV and IR spectra. We also thank the National Science Fund of Bulgaria, Project RNF01/0110, and the Science Fund of the University of Plovdiv, RNI 09-HF-002, for the support. 7. References 1. F. Zuccarello, G. Buemi, C. Gandolfo, A. Contino, Spectroc-him. Acta Part A 2003, 59, 139-151. 2. A. Gringauz, in: Introduction to Medicinal Chemistry - How Drugs Act and Why, Wiley-VCH, New York, 1997. 3. S. R. Marder, D. N. Beratan, L.-T. Cheng, Science 1991, 252, 103-106. 4. (a) S. R. Marder, L.-T. Cheng, B. G. Tiemann, A. C. Friedli, M. Blanchard-Desce, J. W. Perry, J. Skindh0j, Science 1994, 263, 511-514.; (b) M. Blanchard-Desce, V. Alain, P. V. Bed-worth, S. R. Marder, A. Fort, C. Runser, M. Barzoukas, S. Lebus, R. Wortmann, Chem. Eur. J. 1997, 3, 1091-1104. 5. J. Adamson, B. J. Coe, H. L. Grassam, J. C. Jeffery, S. J. Coles, M. B. Hursthouse, J. Chem. Soc., Perkin Trans. 1999, 1, 2483-2488. 6. E. Mendez, M. F. Cerda, J. S. Gancheff, J. Torres, C. Kremer, J. Castiglioni, M. Keininger, O. N. Ventura, J. Phys. Chem. C 2007, 111, 3369-3383. 7. M. R. Chierotti, L. Ferrero, N. Garino, R. Gobetto,L. Pelle-grino, D. Braga, F. Grepioni,L. Maini, Chem. Eur. J. 2010, 16, 4347-4358. 8. R. K. Goel, C. Gupta, S. P. Gupta, Ind. J. Pure. Appl. Chem. 1985, 23, 344-350. 9. M. S. G. Tasende, M. I. S. Gimeno, S. Sanchez, J. S. Casas, J. Sordo, J. Organomet. Chem. 1990, 390, 293-300. 10. S. Millefiori, A. Millefiori, J. Heterocyclic Chem. 1989, 26, 639-644. 11. R. Martos-Calvente,V. A. de la Pena O'Shea, J. M. CamposMartin, J. L. G. Fierro, J. Phys. Chem. A 2003, 107, 74907495. 12. B. Chmura, M. Rode, A. Sobolewski, L. Lapinski, M. Nowak, J. Phys. Chem. A 2008, 112, 13655-13661. 13. A. L. Sobolewski, Chem. Phys. Lett. 1993, 211, 293-299. 14. V. B. Delchev, A. L. Sobolewski, W. Domcke, Phys. Chem. Chem. Phys. 2010, 12, 5007-5015. 15. T. H. Dunning, Jr., J. Chem. Phys. 1989, 90, 1007-1014. 16. R. A. Kendall, T. H. Dunning, Jr., R. J. Harrison, J. Chem. Phys. 1992, 96, 6796-6807. 17. Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennuc-ci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Na-katsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Strat-mann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakka- ra, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004. 18. R. Ahlrichs, M. Baer, M. Haeser, H. Horn, C. Koelmel. Chem. Phys. Lett. 1989, 162, 165-169. 19. C. Haettig, F. Weigend, J. Chem. Phys. 2000, 113, 51545162. Povzetek Predstavljamo teoretične in eksperimentalne raziskave, ki so nam omogočile študij mehanizma fotoinducirane tavtomerije 2-tiobarbiturne kisline (TBA). Obsevanje raztopine TBA v polarnem aprotičnem topilu z UV svetlobo (z maksimumom pri 366 nm) je pokazalo, da poteče okso-hidroksi fotoizomerizacija iz triketo oblike TBA v hidroksi-iminski tavtomer. Študij mehanizma (TD DFT) fotoinducirane disociacije NH in OH v keto in enolnem tavtomeru je pokazal, da odcep protona iz triketo tavtomera poteka preko svetlega 'nSa* vzbujenega stanja; v hidroksi-iminskem tavtomeru pa je mehanizem gnan z odbojnim 'na* vzbujenim stanjem. Relaksacija vzbujenega stanja poteka preko nizko energijskega So-Sj koničnega presečišča. Electronic supporting information Photoinduced Tautomerism of 2-thiobarbituric Acid Studied by Theoretical and Experimental Methods R. Bakalska and V. B. Delchev Table S1. Calculated CC2 vertical excitation energies of the tautomeric forms of thiobar-bituric acid K E eV nm eV nm 1nSn* (A") 3.489 356 1nSn* (A") 2.842 437 1n0n* (A") 4.728 262 1nn* (A') 4.489 276 1nn (A') 4.729 262 1non* (A") 4.747 261 1non* (A") 5.081 244 ^n* (A") 5.083 244 1nSn* (A") 6.272 198 1nSn* (A") 5.651 220 1nn (A') 5.586 222 Xa* (A') 5.573 223 1nSo* (A') 5.822 213 1nn* (A') 5.345 232 1nn (A') 6.726 184 1nn* (A') 6.245 199 1no* (A") 6.208 200 1nSa* (A') 6.393 194 E1 E2 1non* (A") 3.833 324 1nSn* (A") 3.932 316 1an (A") 4.828 257 1nSn* (A") 5.744 216 1nn (A') 5.076 244 1nn* (A') 4.680 265 1nNn* (A") 5.193 239 1nn* (A') 5.084 244 1nn (A') 5.794 214 1na* (A") 5.439 228 1no* (A") 5.866 212 1nSa* (A') 5.675 219 1noO* (A') 5.905 210 1non* (A") 4.991 249 ^rc* (A") 6.185 201 1nn* (A') 5.592 222 1noO* (A') 6.353 195 1non* (A") 5.552 234 E3 E4 ^rc* (A") 3.543 350 1non* (A") 4.441 279 ^rc* (A") 4.340 286 1non* (A") 4.773 260 1KK (A') 4.533 274 1nn* (A') 5.129 242 1KK (A') 5.342 232 1na* (A") 5.396 230 ^rc* (A") 5.724 217 1noa* (A') 5.251 236 ^rc* (A") 5.652 220 1na* (A") 5.633 220 1no* (A") 6.203 200 1na* (A") 5.826 213 ^a* (A') 5.893 211 1noa* (A') 5.634 220 1KK (A') 6.780 183 1nn* (A') 5.735 216 Table S2. Adiabatic excitation energies (CC2) of the tautomeric forms of thiobarbituric acid Optimized K E E1 state State eV nm State eV nm State eV nm W (A') 1 * nsn 2.730 455 Cannot be optimized 1* non 1.822 681 1 * 1nn 3.390 366 1* %% 2.817 441 i * non 4.511 275 1* <5% 3.907 318 1* 1cn 4.880 254 1* 1noO 4.041 307 'nSn* (A") 1* nsn 3.023 410 i * nsn 2.075 598 1* non 1.335 930 for K and E 1* 1nn 4.097 303 1* %% 3.487 356 1* %% 3.158 393 1n0n* (A") 1* non 4.500 276 1* <5% 4.409 281 1* 1noO 3.573 347 for E1 1* 1cn 4.927 252 1* non 4.848 256 1* non 3.901 318 E2 E3 E4 W (A') 1* nsn 3.186 389 1 * non 1.610 771 1* non 1.270 977 1* 1nn 3.673 338 1* %% 2.189 567 1* %% 2.284 543 1* 1nn 4.779 260 1* nNn 2.876 431 1* 1noO 2.839 437 1* nsn 4.818 258 1* non 4.490 276 1* non 2.878 431 'nSn* (A") 1* nsn 3.345 371 1* non 0.725 1712 1* non 1.178 1053 forS E2 1* 1nn 4.057 306 1* %% 2.176 570 1* %% 2.289 542 1n0n* (A") 1* non 4.870 255 1* <5% 3.087 402 1* non 2.679 463 for E3 and E4 1* lnn 4.900 253 1* 1noO 3.521 352 1 * 1noO 2.691 461 Figurs S1. Selected molecular orbitals (optimized at the CC2 level) of the oxo form (K) of TBA involved in the electron transitions