UV-Induced Hydrogen-Atom Transfer in 3,6-Dithiopyridazine and in

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UV-Induced Hydrogen-Atom Transfer in 3,6-Dithiopyridazine and in Model Compounds 2-Thiopyridine and 3-Thiopyridazine Hanna Rostkowska,† Leszek Lapinski,† Igor Reva,‡ Bruno J. A. N. Almeida,‡ Maciej J. Nowak,*,† and Rui Fausto‡ † ‡

Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

bS Supporting Information ABSTRACT: 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 3,6dithiopyridazine. 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.

’ INTRODUCTION Photoinduced hydrogen-atom-detachment and hydrogenatom-transfer processes, driven by repulsive πσ* states, constitute an important group of photochemical reactions.1 For compounds with no internal hydrogen bonding, intramolecular hydrogen-atom-transfer processes2,3 induced by UV excitation of matrix-isolated molecules have been observed since 1988, though the mechanisms governing such occurrences remained unknown until recent years. The breakthrough came with the theoretical work of Sobolewski et al.4 on the role of πσ* states in photoinduced hydrogen-atom detachment from N H and O H groups. Thereafter, the PIDA (PhotoInduced Detachment Attachment) mechanism, involving a key role of πσ* states, was formulated to explain phototautomeric, hydrogen-atom-transfer reactions in compounds such as 4-pyrimidinone.5 According to the theoretical model,5 dissociation of H-atom after electronic excitation to πσ* state is the first step of PIDA-type processes. The number of reports on UV induced N H bond fission (in compounds such as pyrrole) or O H bond fission (in compounds such as phenol) is rapidly growing (see the review in ref 1). In these processes, H-atom loss occurs on the dissociative potential-energy surfaces of excited states with an electron promoted to σ* orbital. Recently, UV-induced hydrogenatom detachment from the S H group has been observed for jet-cooled thiophenol and related mercapto compounds.6 9 r 2011 American Chemical Society

Intramolecular hydrogen-atom-transfer processes, converting thione forms into the respective thiol tautomers, were observed for several matrix-isolated thione compounds, such as 4-thiopyrimidine, 3-thiopyridazine, 2-thiopyridine, and bismuthiol.10 12 These photoprocesses were analogous to the UV induced oxo f hydroxy reactions occurring in compounds with adjacent CdO and N H groups.2,3,13,14 It seems very likely that the thione f thiol phototautomeric reactions, involving (as a first step) hydrogen-atom detachment from the N H group, are also governed by the repulsive πσ* and/or nσ* states. For 2,4dithiouracil, isolated in low-temperature matrixes, UV-irradiation induces transfer of two hydrogen atoms.15,16 This photoisomerization reaction converts the dithione form of the compound into the dithiol tautomer. 3,6-Dithiopyridazine, being the title compound of the present investigation, is an isomer of 2,4-dithiouracil. The two heterocyclic compounds differ only by switching the positions within one pair of N H and CdS groups. However, such a structural difference leads to direct vicinity of two nitrogen atoms in the heterocyclic ring of 3,6-dithiopyridazine. This can substantially influence the relative energies of tautomeric forms of this Received: June 24, 2011 Revised: September 6, 2011 Published: September 19, 2011 12142

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The Journal of Physical Chemistry A compound and the structure actually adopted by monomers of this species. As a consequence, UV-induced transformations of matrix-isolated monomers of 3,6-dithiopyridazine can also be affected. In the current work, the structure of most stable form(s) of 3,6-dithiopyridazine as well as the phototransformations of monomers of this compound were studied using the matrixisolation method. The experimental investigation was supported by quantum chemical calculations.

’ EXPERIMENTAL SECTION 3,6-Dithiopyridazine was synthesized from 3,6-dichloropyridazine (TCI Europe) and thiourea (Aldrich) according to the procedures given in refs 17 and 18. 3-Thiopyridazine was synthesized by action of P2S5 on 3-pyridazinone (Aldrich), as described in ref 19. 2-Thiopyridine was a commercial product supplied by Aldrich. Prior to matrix experiments, the compounds were purified by vacuum sublimation. To prepare Ar matrixes containing isolated monomers of the studied species, a solid sample of 3,6-dithiopyridazine, 3-thiopyridazine, or 2-thiopyridine was heated in a miniature glass furnace placed in the vacuum chamber of a helium-cooled cryostat. Low-temperature Ar matrixes were formed by codeposition of the compound vapors and argon onto a CsI window cooled to 12 K. Argon matrix gas was of spectral purity (N60), as supplied by Air Liquide. The IR spectra were recorded in the 4000 400 cm 1 range, with 0.5 cm 1 resolution, using a Thermo Nicolet 670 FTIR spectrometer equipped with a KBr beam splitter and DTGS detector. Matrixes were irradiated with light from 200W high-pressure mercury or xenon mercury lamp fitted with a water filter and WG335 or UG11 cutoff filters (Schott) transmitting light with wavelengths longer than 335 or 275 nm, respectively. In some experiments, matrixes were irradiated with the frequency-doubled signal beam of the Quanta-Ray MOPO-SL pulsed (10 ns) optical parametric oscillator (fwhm ∼ 0.2 cm 1, repetition rate 10 Hz, pulse energy ∼ 2 mJ at 385 nm) pumped with a pulsed Nd: YAG laser. ’ COMPUTATIONAL SECTION The geometries of the isomeric forms of the compounds considered in the current work (see the structures presented in Tables 1 and 2 and Tables S1 and S2 in the Supporting Information) were fully optimized using the density functional method DFT(B3LYP) with the Becke’s three-parameter exchange functional 20 and the Lee, Yang, Parr correlation functional.21 The 6-311++G(2d,p) basis set was applied in these calculations. The harmonic vibrational frequencies and IR intensities were calculated at the DFT(B3LYP)/6-311++G(2d,p) level. To correct for the systematic shortcomings of the applied methodology (mainly for anharmonicity), the predicted vibrational wavenumbers were scaled down by a single factor of 0.98. The theoretical normal modes of the 3,6-dithiopyridazine and 3-thiopyridazine tautomers were analyzed by carrying out the potential energy distribution (PED) calculations, performed according to the procedure described in refs 22 and 23. The sets of internal coordinates used in the PED analysis were defined following the recommendations of Pulay et al.24 These coordinates are listed in Tables S3 S6 (Supporting Information). Cartesian force constants were transformed into the force constants with respect to the molecule fixed internal coordinates. Potential energy distribution matrices have been calculated, and the elements of

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Table 1. Relative Electronic (ΔEel), Zero-Point Vibrational (ΔZPE), and Total (ΔEtotal = ΔEel + ΔZPE) Energies (kJ mol 1) of Isomeric Forms of 3,6-Dithiopyridazinea

ΔEel calculated using the QCISD/6-31++G(d,p) method at geometries optimized at the same level; ΔZPE calculated at the DFT(B3LYP)/ 6-311++G(2d,p) level.

a

these matrices greater than 10% are given in Tables S7 S10 (Supporting Information). The electronic energies (Eel) of all the isomers considered in the current work were calculated using the quadratic configuration interaction method with single and double excitations (QCISD)25 and the 6-31++G(d,p) basis set. These calculations were carried out for geometries optimized at the QCISD/6-31+ +G(d,p) level as well as at the DFT(B3LYP)/6-311++G(2d,p) level. Every structure (see Tables 1, 2, S1, and S2) optimized at the DFT(B3LYP) or QCISD level was found to correspond to a potential energy minimum. All the calculations were performed with the Gaussian 03 program.26

’ RESULTS AND DISCUSSION Prototropic Tautomerism. The relative energies of the thione-thiol Itntl, dithiol Idtl and dithione Idtn tautomeric forms of 3,6-dithiopyridazine were calculated at the QCISD level. The results of these calculations (presented in Table 1 and in Table S1 in the Supporting Information) suggest that the thione thiol form Itntl should be significantly more stable than the dithiol form Idtl (higher in energy by 13.5 kJ mol 1) and than the dithione form Idtn (higher in energy by 39.6 kJ mol 1). The geometry of this latter form was predicted to be nonplanar, with the C2 symmetry and the optimized values of the H N N H dihedral angle equal to 18.9° (DFT) or 43.0° (QCISD) and of the C N N C dihedral angle equal to 7.7° (DFT) or 25.5° (QCISD). Provided that the QCISD calculations correctly predict the relative energies of 3,6-dithiopyridazine tautomers, the thione thiol form Itntl should be populated nearly exclusively in the gas phase and, as a consequence, trapped in low-temperature matrixes. The experimental matrix-isolation study of 3,6-dithiopyridazine carried out within the current work demonstrates that, in agreement with the QCISD prediction, the population of the thione thiol tautomer Itntl highly dominates in low-temperature Ar matrixes. In the experimental IR spectrum of 3,6dithiopyridazine monomers (Figure 1B), the characteristic bands due to the stretching vibrations of the N H group (νNH) and of the S H group (νSH) were observed at 3403 and 2610 cm 1, respectively. The whole FTIR spectrum of 3,6-dithiopyridazine in an Ar matrix is very well reproduced by the spectrum calculated at the DFT(B3LYP)/6-311++G(2d,p) level for the thione thiol tautomeric form (Figure 1A). This shows that 12143

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Table 2. Relative Electronic (ΔEel), Zero-Point Vibrational (ΔZPE), and Total (ΔEtotal = ΔEel + ΔZPE) Energies (kJ mol 1) of Isomeric Forms of 2-Thiopyridine and 3-Thiopyridazinea

a

Energy of 2-thiopyridine isomer IItn was calculated with respect to the IItl form; Energy of 3-thiopyridazine isomer IIItl was calculated with respect to the IIItn form. †ΔEel calculated using the QCISD/6-31++G(d,p) method at geometries optimized at the same level. ‡ΔZPE calculated at the DFT(B3LYP)/ 6-311++G(2d,p) level. Note the significant difference (10 13 kJ mol 1) in the zero-point vibrational energies ZPE of the thione and thiol tautomers. This ZPE difference always contributes to the relative stabilization of the thiol form.

the Itntl form of 3,6-dithiopyridazine indeed strongly dominates in the matrix. The results of UV irradiation, described in the next section, support the conclusion that the thione thiol form of 3,6-dithiopyridazine is nearly exclusively populated in an Ar matrix. Although it has been recently demonstrated13,27 that the results of the QCISD calculations reproduce the experimental assessments of relative energies of the oxo and hydroxy tautomers of 2-pyridinone (and related compounds) with accuracy not worse than 2 3 kJ mol 1, no analogous tests have been performed so far for the relative energies of thione and thiol tautomers. Because the tautomeric equilibrium in 3,6-dithiopyridazine is strongly shifted in favor of one of the forms, it is not possible to accurately assess the experimental ratio of tautomers in this compound. Therefore, this case cannot serve as a good test of the accuracy of the QCISD calculations. To carry out such a test, we calculated the relative energies of the thiol IItl and thione IItn tautomers of 2-thiopyridine. For this archetype compound, the energy difference predicted at the QCISD/6-31++G(d,p) level (10.9 kJ mol 1 in favor of form IItl, see Tables 2 and S2) is nearly the same as the result (10.9 kJ mol 1) of the previous CCSD(T) calculation.28 These theoretical values are in nice agreement with the experimental assessment (10.0 ( 1.5 kJ mol 1 in favor of the thiol form IItl).11,29 31 The experimental estimate of relative energies of the thiol and thione tautomers of 2-thiopyridine was based on microwave,31 X-ray core-level photoemission,31 ion cyclotron resonance,29 and matrix-isolation IR spectroscopy.11 In the latter case, the IR spectra of thiol and thione tautomeric forms were separated using the photochemical effect shown in Figures 2 and S1. Comparison with 2-thiopyridine, a compound where the NdC SH fragment preferably adopts the thiol form, does not explain why in 3,6 dithiopyridazine (a compound with two such fragments in a six-membered ring) one of them should adopt the HN CdS structure and the other the NdC SH structure. Furthermore, comparison with 2,4-dithiouracil15,16 (adopting exclusively the dithione form) does not suggest either that the thione-thiol form of 3,6-dithiopyridazine should be so much energetically favored over other tautomers of this molecule. The currently considered tautomerism of 3,6-dithiopyridazine seems to be yet another example of tautomerism in heterocyclic compounds with two nitrogen atoms at the vicinal positions. When none of the two nitrogen atoms is protonated (e.g., form

Idtl of 3,6-dithiopyridazine), the repulsive forces between the adjacent lone-electron pairs destabilize the system. If both nitrogen atoms are protonated (e.g., form Idtn of 3,6-dithiopyridazine), the system is destabilized by repulsive forces between the positively loaded hydrogen atoms (because of the latter effect form Idtn is nonplanar). Only in the thione thiol tautomer Itntl these destabilizing, repulsive forces are replaced by an attractive interaction between the lone-electron pair at one of the nitrogen atoms and the positively-loaded hydrogen atom attached to the other nitrogen atom. The latter interaction significantly contributes to stabilization of the system adopting the Itntl structure. For the analogous reason, the oxo-hydroxy tautomer is the most stable form of 3-hydroxy-6(1H)-pyridazinone32 and the thione thiol form is the only thermally populated form of bismuthiol (5-mercapto-1,3,4-thiadiazole-2-thione).12 Similar interactions govern also the tautomerism of another model compound, 3-thiopyridazine.10 In this compound, the presence of an additional (in comparison with 2-thiopyridine) nitrogen atom in the ring significantly shifts the tautomeric equilibrium in favor of the thione form IIItn. According to the QCISD calculations (see Tables 2 and S2), this form is lower in energy by 18.3 kJ mol 1 than the thiol form IIItl. Hence, IIItn should be the only form populated in the gas phase as well as in low-temperature matrixes. Experimental results confirm this theoretical prediction. The IR spectrum of the compound isolated in an argon matrix consists only of the bands due to the thione IIItn form (see Figure 3B,A). No bands owing to the thiol IIItl form were found in the experimental spectrum. Summarizing, in all the molecular systems mentioned above, tautomers with just one of the vicinal nitrogen atoms being protonated are strongly energetically favored, in comparison to forms with none of the nitrogen atoms protonated or forms with both nitrogen atoms protonated. UV-Induced Hydrogen-Atom Transfer. Phototautomeric reaction transforming the thione form into the thiol tautomer (Scheme 1) was observed for the model compound, 2-thiopyridine. Due to the very low initial population of the thione tautomer IItn (thione/thiol ratio equal to 1:20, as experimentally estimated in the current work), the results of this phototransformation were not very pronounced. Upon UV (λ > 335 nm) irradiation, very weak bands due to form IItn disappeared, whereas the bands originating from the dominant thiol form IItl increased in intensity (see Figures 2 and S1 and ref 11). 12144

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Figure 1. Infrared spectra of 3,6-dithiopyridazine isolated in an Ar matrix recorded (B) after deposition of the matrix and (C) after 90 min of UV (λ > 335 nm) irradiation; compared with (A) the theoretical spectrum of the thione thiol Itntl form; (D) the theoretical spectrum of the dithiol Idtl form. The calculations were carried out at the DFT(B3LYP)/6-311++G(2d,p) level. The computed wavenumbers were scaled by the factor of 0.98.

Figure 2. Fragments of the infrared spectra of 2-thiopyridine isolated in an Ar matrix recorded (A) after deposition of the matrix, (B) after 20 min of UV (λ > 335 nm) irradiation, (C) difference spectrum: trace (B) minus trace (A); compared with (D) the theoretical spectrum of the thiol IItl form; and (E) the theoretical spectrum of the thione IItn form. The calculations were carried out at the DFT(B3LYP)/6-311++G(2d,p) level. The computed wavenumbers were scaled by the factor of 0.98. Calculated intensities of the bands in the theoretical spectrum (E) of the thione IItn form were multiplied by 0.05.

More spectacular effects were observed for another model compound, 3-thiopyridazine. After deposition of the matrix, monomers of this substance isolated in an Ar matrix adopt solely the thione form IIItn. The spectrum obtained at this stage of experiment fits well the spectrum predicted theoretically for form IIItn. No bands due to the thiol tautomer IIItl (Scheme 1) could be detected. When exposed to UV radiation, isolated molecules

of 3-thiopyridazine convert nearly totally into a photoproduct (see Figure 3 and ref 10). In the current work, the photoproduced species was unquestionably identified as the thiol form IIItl. This identification was based on a very good agreement between the experimental spectrum of the photoproduct and the theoretical spectrum calculated for IIItl, as presented in Figure 3C,D and in Table S10. 12145

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Figure 3. Infrared spectra of 3-thiopyridazine isolated in an Ar matrix recorded (B) after deposition of the matrix and (C) after 45 min of UV (λ > 335 nm) irradiation; compared with (A), the theoretical spectrum of the thione IIItn form; (D) the theoretical spectrum of the thiol IIItl form. The calculations were carried out at the DFT(B3LYP)/6-311++G(2d,p) level. The computed wavenumbers were scaled by the factor of 0.98.

Scheme 1. Phototautomeric Reactions Observed for 2-Thiopyridine, 3-Thiopyridazine, and 3,6-Dithiopyridazine

Monomers of 3,6-dithiopyridazine isolated in Ar matrixes were exposed to broadband UV (λ > 335 nm) light or, in separate experiments, to monochromatic (λ = 385 nm) laser light. The effects induced by (λ > 335 nm) and by (λ = 385 nm) irradiation were the same. The population of the thione thiol form Itntl, dominating very strongly in the matrix before any irradiation, was systematically diminishing upon UV excitation. This was revealed by a systematic decrease of the whole initial IR spectrum, including the characteristic band at 3403 cm 1 due to the stretching

vibration of the N H group (see Figure 1). In the IR spectrum emerging upon UV irradiation, no new band appeared in the spectral range close to 3400 cm 1, where IR absorptions due to NH stretching vibrations are expected. This indicates that there are no N H groups in the structure of the photoproduct. In the spectral region 2620 2590 cm 1, where the bands due to SH stretching vibrations are expected, the initial absorption at 2610 cm 1 disappeared upon UV irradiation, while a new band appeared at 2606 cm 1 (Figure 4). The new band is twice as intense as the band at 2610 cm 1 in the spectrum of the reactant. This is in agreement with the predicted relative intensities of these two bands in the spectra of forms Itntl and Idtl (Tables S7 and S8). The “fingerprint” region of the IR spectrum of the photoproduct has a very characteristic pattern of bands. It is dominated by two strong bands emerging upon UV irradiation at 1389 and 1157 cm 1. The same characteristic spectral pattern, with two intense bands at 1393 and 1146 cm 1, was predicted in the theoretical spectrum calculated for the dithiol tautomer Idtl. According to the calculations, the band at 1389 cm 1 is due to the antisymmetric stretching vibrations of the two C N bonds (coupled with bending vibrations of C H groups), whereas the band at 1157 cm 1 originates from the triangular deformation of the ring (coupled with the antisymmetric stretching vibrations of the two C S bonds and the bending vibrations of C H groups), see Table S8. Comparison of the experimental spectrum recorded after UV irradiation of matrix-isolated 3,6-dithiopyridazine and the theoretical spectrum of form Idtl demonstrates without any doubt that the dithiol tautomer of the compound is the photogenerated species. In the initial spectrum (recorded before any irradiation) only extremely low-intensity absorptions at 1389 and 1157 cm 1 can be ascribed to form Idtl. This indicates that, in the freshly deposited matrix, the relative amount of the dithiol form Idtl must be lower than 1%. 12146

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Figure 4. Infrared spectra in the range of 2625 2595 cm 1 of 3,6dithiopyridazine isolated in an Ar matrix: (black) after deposition of the matrix; (red) after 90 min of UV (λ > 335 nm) irradiation.

Figure 6. Partial reversibility of the thione a thiol phototautomerism in 3-thiopyridazine. Portion of the infrared spectra of 3-thiopyridazine isolated in an Ar matrix recorded: (C) after deposition of the matrix; (A) after 45 min of UV (λ > 335 nm) irradiation; (B) the spectrum recorded after subsequent 45 min of UV (λ > 275 nm) irradiation minus spectrum (A).

Figure 5. Partial reversibility of the thione a thiol phototautomerism in 3,6-dithiopyridazine. Portion of the infrared spectra of 3,6-dithiopyridazine isolated in an Ar matrix recorded: (C) after deposition of the matrix; (A) after 90 min of UV (λ > 335 nm) irradiation; (B) the spectrum recorded after subsequent 45 min of UV (λ > 275 nm) irradiation minus spectrum (A).

After more than 95% of the thione thiol form Itntl of 3,6dithiopyridazine had been transformed, upon λ > 335 nm (or λ = 385 nm) irradiation, into the dithiol form Idtl, the matrix was further exposed to UV (λ > 275 nm) light. This shorterwavelength irradiation led to partial repopulation of form Itntl, as shown in Figures 5 and S2 in the Supporting Information. Thereby the partial photoreversibility of the UV-induced thione a thiol transformation in 3,6-dithiopyridazine was demonstrated. Analogous partial photoreversibility was also observed for 3-thiopyridazine (see Figure 6). Monomers of 3-thiopyridazine, frozen in an argon matrix and irradiated with UV (λ > 335 nm) light, were subsequently exposed to shorter-wavelength UV (λ > 275 nm) radiation. The effect of the second (λ > 275 nm) irradiation was reverse to the effect of the first irradiation: the IR bands due to form IIItl (that was produced upon λ > 335 nm irradiation) diminished, whereas the bands due to form IIItn (that was consumed upon λ > 335 nm irradiation) increased in intensity (see Figure 6B). The effects found for 3,6-dithiopyridazine and for 3-thiopyridazine provide the first experimental evidence of the reversibility of the thione a thiol phototautomerism. For small thioamide molecules such as thiourea and dithiooxamide, a back thiol f thione tunneling in the dark (following the

UV-induced thione f thiol phototransformation) was previously observed.33,34 However, no analogous tunneling occurred when the matrix-isolated dithiol form Idtl of 3,6-dithiopyridazine, generated upon UV (λ > 335 nm) irradiation, was kept in darkness at 10 K for several hours. No tunneling thiol f thione conversion was observed either for the photoproduced thiol form IIItl of 3-thiopyridazine. Very high barriers calculated for the thiol f thione groundstate tunneling (136 kJ mol 1 for the Idtl f Itntl conversion and 131 kJ mol 1 for the IIItl f IIItn conversion, see Tables S1 and S2) explain these observations. Characteristic IR Bands of the Thiol Forms of Heterocyclic Compounds. A thione form of a heterocyclic compound, with the HN CdS fragment in the ring, can be easily identified using IR spectroscopy. In the spectra of such thione tautomers, a relatively intense infrared band due to NH stretching vibration always appears at a characteristic position near 3400 cm 1. Usually, it is more difficult to identify thiol tautomers of such compounds, with NdC SH fragments in the molecules. Although the bands due to SH stretching vibrations (νSH) are always placed near the very characteristic frequency 2600 cm 1, they are more difficult to detect because of their very low infrared intensity. The integral absorption coefficient of a νSH band is about 20 times smaller than that of a νNH band. As far as 3,6-dithiopyridazine is concerned, the νSH bands were found in the spectrum of the initial thione-thiol form (at 2610 cm 1) and in the spectrum of the UV-generated dithiol form (at 2606 cm 1). According to the theoretical calculations given in Table S8, this latter band should be due to the symmetric stretching vibrations of two S H groups (νsSH). In the IR spectra of the thiol tautomers of 2-thiopyridine and 3-thiopyridazine isolated in Ar matrixes, the νSH bands were observed at 2609 and 2600 cm 1, respectively. 12147

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Figure 7. Absorption bands due to the βSH vibrations found in the IR spectra of: (A) the thiol form of 2-thiopyridine; (B) the thiol form of 3-thiopyridazine; (C) the thione thiol form of 3,6-dithiopyridazine; and (D) the dithiol form of 3,6-dithiopyridazine, isolated in low-temperature Ar matrixes, compared with the corresponding bands in the theoretical spectra calculated at the DFT(B3LYP)/6-311++G(2d,p) level. The computed wavenumbers were scaled by the factor of 0.98.

Not only the bands due to the stretching vibrations of S H groups, but also those originating from in-plane bending C S H vibrations (βSH) are characteristic features present in the IR spectra of all thiol heterocyclic molecules examined within this study. The βSH bands appear at ∼890 cm 1 and are usually well separated from other bands (see Figure 7). In the spectra of the thiol forms of 2-thiopyridine and 3-thiopyridazine, the characteristic βSH bands were found at 884 and 889 cm 1, respectively (see Figure 7). For other heterocyclic thiol molecules, such as 2-thiopyrimidine,35 4-thiopyrimidine,10 and 2-thiopyrazine, the characteristic bands ascribed to the deformational vibrations βSH appear also in the same spectral range, at 907, 894, and 886 cm 1. In the spectrum of the thione thiol form of 3,6-dithiopyridazine, the βSH band was observed at 889 cm 1, whereas in the spectrum of the photogenerated dithiol tautomer, new βSH bands appeared at 897 and 882 cm 1. These latter bands were assigned to the antisymmetric βaSH and symmetric βsSH bending motions, respectively (see Table S8). The positions of the bands due to βSH vibrations should be sensitive to interactions with the lone-electron pairs of vicinal heteroatoms (see e.g. the discussion in ref 12). For IItl(a) form of 2-thiopyridine (see Table S2), where the S H group interacts with the lone-electron pair of the nitrogen atom, the βSH band appears at 884 cm 1. Such interaction is absent in the case of rotamer IItl(b). According to the DFT(B3LYP)/6-311++G(2d,p) calculations, the βSH band should appear in the IR spectrum of IItl(b) at a wavenumber higher by some 40 cm 1. However, no such band appears in the experimental spectrum of matrix isolated 2-thiopyridine. This demonstrates that rotamer IItl(b) is not stabilized in a low-temperature matrix. The rotamer IItl(b) is theoretically predicted (Table S2) to be higher in energy only by 6.0 kJ mol 1, with respect to the most stable IItl(a) form. Moreover, IItl(b) was found by means of microwave spectroscopy31 to be populated in the gas phase, but upon freezing into a low-temperature matrix, this form must have converted into the lower-energy IItl(a) form. This process was allowed by a low barrier (∼9.6 kJ mol 1, see Table S2) for the IItl(b) f IItl(a) conversion. Transformation of the higher-energy cis conformer of hydroquinone into the lower-energy trans form was observed36 for the compound isolated in an argon matrix at 16 K, although the barrier separating these two forms was as high as 10.8 kJ mol 1. Analogously, any signatures of the thione thiol rotamer Itntl(b) were not observed either in the IR spectra of 3,6dithiopyridazine recorded after deposition of the matrix or in the spectra recorded after UV irradiation. According to the

calculations, the Itntl(b) f Itntl(a) conversion should be very easy, since the barrier for this process is very low (∼2.9 kJ mol 1, see Table S1).

’ CONCLUSIONS 3,6-Dithiopyridazine monomers isolated in low-temperature Ar matrix were found to adopt the thione thiol tautomeric form. This form was theoretically predicted (at the QCISD level) to be the most stable tautomer of the compound. Only in the thione thiol form of 3,6-dithiopyridazine there is a stabilizing interaction between the positively loaded hydrogen atom of the N H group and the lone-electron pair of the nitrogen atom in the vicinal position. In other tautomers, repulsive interactions between two positively loaded hydrogen atoms of the N H groups (dithione form) or between two lone-electron pairs of vicinal nitrogen atoms (dithiol form) considerably destabilize the system. The reliability of QCISD predictions of relative energies of thione and thiol tautomers of heterocyclic compounds was tested on the archetype 2-thiopyridine molecule. The experimental energy difference of 10.0 ( 1.5 kJ mol 1 (in favor of the thiol form) was very well reproduced by the results of QCISD calculation. Similar relative energy assessment was obtained in the CCSD(T) calculation, 28 whereas the predictions of the popular MP2 30,31 and DFT(B3LYP) methods are usually significantly inaccurate (21 26 and 3 kJ mol 1 , respectively). Irradiation of matrix-isolated 3,6-dithiopyridazine monomers with broadband (λ > 335 nm) or monochromatic λ = 385 nm light led to transformation of the thione-thiol tautomer into the dithiol form (Scheme 1). This allowed an unprecedented observation and spectral characterization of the dithiol tautomer of 3,6-dithiopyridazine. Subsequent irradiation of the matrix with shorter-wave UV (λ > 275 nm) light induced the thiol f thione phototransformation leading to partial repopulation of the thione thiol (for 3,6-dithiopyridazine) reactant (see Scheme 1). Partial reversibility was also found for the thione a thiol phototransformation in the model compound 3-thiopyridazine. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1, illustrating phototautomeric transformation in 2-thiopyridine; Figure S2, showing partial reversibility of phototautomeric process in 3,6-dithiopyridazine; Tables S1 and S2, providing the calculated relative

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dx.doi.org/10.1021/jp2059563 |J. Phys. Chem. A 2011, 115, 12142–12149

The Journal of Physical Chemistry A energies of isomeric forms of 3,6-dithiopyridazine, 2-thiopyridine, and 3-thiopyridazine; Tables S3 S6, providing the internal coordinates used in the normal-mode analysis for the thione thiol and dithiol tautomers of 3,6-dithiopyridazine, as well as for the thione and thiol tautomers of 3-thiopyridazine; and Tables S7 S10, providing the assignment of the absorption bands in the experimental IR spectra of the thione thiol and dithiol tautomers of 3,6-dithiopyridazine, as well as of the thione and thiol tautomers of 3-thiopyridazine to the theoretically calculated normal modes. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been partially funded by the Portuguese Science Foundation (FCT), under Research Projects PTDC/QUI/ 71203/2006-FCOMP-01-0124-FEDER-007458 and PTDC/ QUI-QUI/111879/2009, cofunded by QREN-COMPETE-UE.

ARTICLE

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