View PDF Version
 
DOI: 10.1039/D4CP02799J (Paper) Phys. Chem. Chem. Phys., 2024, Advance Article

Intramolecular hydrogen-atom tunneling in matrix-isolated heterocyclic compounds: 2-thiouracil and its analogues†

Hanna Rostkowska, Leszek Lapinski* and Maciej J. Nowak*
Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, Warsaw 02-668, Poland. E-mail: mjnow@ifpan.edu.pl

Received 15th July 2024 , Accepted 26th August 2024

First published on 27th August 2024

Abstract

Hydrogen-atom tunneling leading to spontaneous tautomeric conversion in monomeric heterocyclic molecules without intramolecular hydrogen bonds has been experimentally detected for the first time. For monomers of 2-thiouracil, 6-aza-2-thiouracil and 1-methyl-2-thiouracil isolated in low-temperature matrices, higher-energy thiol forms were generated upon UV (λ = 305 nm) excitation of the most stable thione tautomers. When the matrices were subsequently kept in the dark and at low temperature, hydrogen-atom tunneling occurred, leading to the thiol → thione conversion. During this process, the photoproduced thiol form spontaneously converted into the lowest energy thione tautomer.

1 Introduction

Hydrogen-atom tunneling plays an important role in various chemical transformations.1–3 One of the most advantageous experimental techniques for observation of slow spontaneous structural changes involving intramolecular hydrogen-atom tunneling is matrix isolation combined with IR spectroscopy.4 Numerous reports on experimental observations of hydrogen-atom tunneling in matrix-isolated species concern conformational isomerizations in which no chemical bond is broken and no new chemical bond is formed.5–8 Conversion of one tautomeric form of a compound into another requires breaking of a chemical bond and formation of a new chemical bond. Barriers for such transformations are usually very high (of the order of 100 kJ mol−1). Until now, a number of examples of spontaneous transformations of the higher-energy tautomer into the lower-energy tautomeric form have been reported.5,7–18 One group of such tunneling conversions concerns spontaneous transformation of the thiol (or selenol) form of a simple thioamide (selenoamide) into the lower-energy thione (selenone) tautomer. Such transformations have been observed for thiourea, 1-methylthiourea, selenourea, dithiooxamide and thiobenzamide.14–18 For these compounds isolated in low-temperature matrices, the higher-energy thiol, dithiol or selenol forms were generated upon UV excitation of the initially trapped lowest-energy thione or selenone tautomer.

Because of the high barriers (see Table 1), the observed thiol → thione (or selenol → selenone) tunneling processes were slow. For selenourea, the higher-energy selenol tautomer was consumed in such tunneling transformation following the monoexponential decay pattern [n(t) = n(t0)exp(−t/τ)] with a time constant τ of 16 hours. For dithiooxamide, the dithiol → dithione process occurred with a time constant of 18 hours; for thiourea and 1-methylthiourea, the thiol → thione conversions proceeded with time constants of 52 and 19 hours, respectively, whereas for the very slow thiol → thione transformation in thiobenzamide, the time constant was ca 10 days.18 For the simplest thioamides (thioformamide and thioacetamide), the thiol → thione tunneling transformation was too slow to be experimentally detected.7,8 As far as processes involving transfer of a single hydrogen atom are concerned, the time constants of the tunneling transformations show some correlation18 with the theoretically computed heights of the barriers for the thiol → thione or selenol → selenone conversions. So far, all the reported cases of the experimentally observed spontaneous thiol → thione or selenol → selenone transformation were found only for simple thioamides (or selenoamides), where the H–S–C–N or H–Se–C–N fragment, directly involved in hydrogen-atom transfer, was not incorporated into a rigid heterocyclic ring.

Table 1 The heights of the barriers for the thiol → thione transformation in the ground electronic state of heterocyclic thio compounds and simple thioamides. The energies of transition states calculated at the MP2/6-311++G(2d,p) level are given with respect to the energy of the thiol isomer. Experimentally estimated time constants of the spontaneous thiol → thione tunneling process were derived from the infrared spectra of the compounds isolated in Ar matrices
Spontaneous transformation Calculated barrier height (kJ mol−1) Thiol → thione tunneling time constant
image file: d4cp02799j-u1.tif 93 Tunneling
τ = 76 h
τ = 68 h (Ne)
image file: d4cp02799j-u2.tif 88 Tunneling
τ = 1.6 h
image file: d4cp02799j-u3.tif 99 Tunneling
τ = 1160 h
image file: d4cp02799j-u4.tif 107 No tunneling
image file: d4cp02799j-u5.tif 102 No tunneling
image file: d4cp02799j-u6.tif 137 No tunneling
image file: d4cp02799j-u7.tif 95 Tunneling
(ref. 18) τ = 19 h
image file: d4cp02799j-u8.tif 104 Tunneling
(ref. 15) τ = 52 h
image file: d4cp02799j-u9.tif 115 No tunneling

2 Experimental and theoretical methods

2-Thiouracil used in the present study was a commercial product supplied by Aldrich. 6-Aza-2-thiouracil was provided by Angene Chemical Ltd. The 1-methyl-2-thiouracil sample was kindly made available by Professor D. Shugar (University of Warsaw). Low-temperature Ar and Ne matrices were deposited onto a CsI window mounted on the cold finger of a Sumitomo SRDK-408D2 closed-cycle cooler. Infrared spectra were collected using a Thermo Nicolet iS50R FTIR spectrometer. Matrix-isolated molecules were irradiated with quasi-monochromatic (FWHM = 15 nm) UV (λ = 305 nm) light emitted using a 6060 LG Innotek diode (optical power = 100 mW).

Infrared spectra were theoretically simulated within the harmonic approximation at the DFT(B3LYP)/6-311++G(2d,p) level.19–21 Theoretical wavenumbers were scaled by 0.98 (for wavenumbers lower than 2000 cm−1) or by 0.95 (for wavenumbers higher than 2000 cm−1). The heights of the barriers for the thiol → thione transformation in the ground electronic state of the studied thio compounds were calculated at the MP2/6-311++G(2d,p) level.22 In every case, the barrier height was calculated as the highest point of the minimum-energy path of the thiol → thione transition. All calculations were performed using the Gaussian 09 program.23

3 Results and discussion

3.1 Search for intramolecular hydrogen-atom tunneling in monomers of heterocyclic compounds

Within the current work, we searched for experimentally detectable intramolecular hydrogen-atom tunneling that would change a tautomeric form of a heterocyclic compound. For this purpose, a series of experiments were carried out to test whether a spontaneous thiol → thione transformation would occur in matrix-isolated monomers of a heterocyclic compound. In each of these experiments, the higher-energy thiol tautomeric form of a compound was photoproduced by UV excitation of the initially deposited, most stable thione tautomer.24 The attempts to observe spontaneous tautomeric conversions in heterocyclic compounds with five-membered rings such as 2-thioimidazole, 3-thio-1,2,4-triazole, 5-methyl-2-thio-1,3,4-thiadiazole, and 2-thiobenzothiazole and in compounds with six-membered heterocyclic rings such as 3-thiopyridazine and 2,6-dithiopurine (see the structures in Scheme S1, ESI†) have not been successful.

The experiments carried out for these compounds did not reveal any increase, on the time scale of hours, of the population of the lower-energy thione forms at the expense of the population of the photogenerated higher-energy thiol forms. It is possible to understand these negative results by considering the height of the barriers for such processes, ca. 137 kJ mol−1 for compounds with five-membered heterocyclic rings and ca. 102 kJ mol−1 for compounds with six-membered heterocyclic rings, see Table 1. In simple thioamides, the H–S–C–N moiety is flexible, but in heterocyclic thioamides the H–S–C–N fragment is incorporated into a rigid ring. This is another factor that may reduce the probability of hydrogen-atom tunneling in heterocyclic thioamides. Overall, one might come to the conclusion that hydrogen-atom tunneling does not occur (on the time scale of hours or days) for compounds with the thioamide group incorporated into a five- or six-membered heterocyclic ring.

3.2 Hydrogen-atom tunneling in matrix-isolated molecules of 2-thiouracil

The experiments carried out within the current work demonstrated that, for one of the photogenerated thiol forms of matrix-isolated 2-thiouracil, the intramolecular thiol → thione hydrogen-atom tunneling occurs. In low-temperature Ar or Ne matrices, the monomers of 2-thiouracil adopt only the most stable oxo-thione tautomeric form I (see Scheme 1 and the spectra in Fig. S1, ESI†).
image file: d4cp02799j-s1.tif
Scheme 1 Photogeneration of three tautomers of 2-thiouracil, produced upon UV (λ = 305 nm) irradiation of the most stable oxo-thione form I and the spontaneous thiol → thione tunneling transformation converting the photoproduced form III into the lowest energy form I.

Upon excitation with UV (λ = 305 nm) light, two (II and III) oxo-thiol tautomers and the hydroxy-thiol form (IV) were generated (Scheme 1 and Fig. 1a and b).25 Photoproduct II is formed by transfer of an H-atom from the N1 nitrogen atom to the sulfur atom, whereas photoproduct III is generated by transfer of an H-atom from the N3 nitrogen atom to the sulfur atom (Scheme 1). The hydroxy-thiol form IV resulted from the transfer of two hydrogen atoms from N1 and N3 to the sulfur and oxygen atoms.


image file: d4cp02799j-f1.tif
Fig. 1 High-frequency fragment of the infrared spectra of 2-thiouracil monomers isolated in a low-temperature Ar matrix: (a) after deposition of the matrix, (b) after 5 hours of exposure to UV (λ = 305 nm) light, and (c) after subsequent 164 hours when the matrix was kept at 3.5 K and in the dark, compared with the theoretical spectra simulated for (black) the oxo-thione form I, (violet) the oxo-thiol form II, (red) the oxo-thiol form III, and (green) the hydroxy-thiol form IV of the compound.

The barrier for the II → I transition in the ground electronic state was theoretically predicted to be as high as 107 kJ mol−1 (Table 1). This value is slightly higher than the barrier for the thiol → thione conversion calculated for 3-thiopyridazine (102 kJ mol−1), for which no thiol → thione tunneling was detected; hence, one should not expect the II → I tunneling to proceed with a time constant low enough to allow experimental observation of this process. However, the theoretically calculated barrier for the III → I process in a molecule of 2-thiouracil is significantly lower (93 kJ mol−1, see Table 1), suggesting the possibility of the III → I tunneling phenomenon.

And indeed, in accord with this theoretical result, the spontaneous thiol → thione process of hydrogen-atom tunneling was experimentally observed for the thiol tautomer III of 2-thiouracil isolated in low-temperature Ar or Ne matrices. During this process, occurring in monomers isolated in Ar or Ne matrices kept in the dark and at 3.5 K, the population of the lowest energy oxo-thione form I grew at the expense of the decreasing population of the photoproduced oxo-thiol form III (Fig. 1b and c). This is revealed by the growth of the IR bands due to I and the decrease of IR bands due to III, presented in the difference spectra in Fig. 2.


image file: d4cp02799j-f2.tif
Fig. 2 Spectral effects of the hydrogen-atom tunneling transformation converting the photoproduced oxo-thiol form III of 2-thiouracil into the lowest energy oxo-thione form I: (a) the theoretical spectrum simulated for the oxo-thiol form III, (b) the difference spectrum obtained by subtraction of the spectrum recorded after irradiation with UV (λ = 305 nm) light from the spectrum recorded after subsequent 47 hours when the Ne matrix was kept at 3.5 K and in the dark, (c) the difference spectrum obtained by subtraction of the spectrum recorded after irradiation with UV (λ = 305 nm) light from the spectrum recorded after subsequent 164 hours when the Ar matrix was kept at 3.5 K and in the dark, and (d) the experimental spectrum of monomeric 2-thiouracil recorded after deposition of the Ar matrix.

Assignment of the growing spectrum to tautomer I is straightforward; this spectrum is identical with the spectrum of I recorded directly after deposition of a low-temperature matrix. Assignment of the decreasing spectrum to form III seems to be very reliable; this spectrum is reproduced well by the theoretical spectrum computed for III (Fig. 2a–c and Table S1, ESI†). The time constant (τ) of the spontaneous III → I process was determined by fitting the I(t) = I(t0) exp (−t/τ) function to the experimental points I(t0), I(t1), I(t2), I(t3)…, where I(t) is the intensity of an experimental IR band due to form III, measured after t hours of keeping the matrix in the dark (see Fig. S2, ESI†). The obtained time constants τ = 76 hours (Ar) and τ = 68 hours (Ne) are similar to each other. Small dependence of these time constants on the solid–noble gas environment demonstrates that this thiol → thione tunneling transformation is virtually an intramolecular process.15

During the whole period of observation of the III → I conversion, no changes were found in the population of the other photoproducts II and IV. Apparently, no spontaneous tautomeric conversions occur for oxo-thiol tautomer II and hydroxy-thiol form IV.

3.3 Hydrogen-atom tunneling in matrix-isolated molecules of 1-methyl-2-thiouracil

The occurrence of the spontaneous thiol → thione hydrogen-atom tunneling was also investigated for 1-methyl-2-thiouracil. Monomers of this compound isolated in an Ar matrix (Fig. S3, ESI†) adopt exclusively the most stable oxo-thione tautomeric form V.26 Upon excitation of matrix-isolated V with UV (λ = 305 nm) light, the oxo-thiol form VI was photogenerated together with a small amount of the hydroxy-thione form VII (Scheme 2 and Fig. S4, S5, ESI†).25 The photoinduced thione → thiol process did not lead to complete transformation of the reactant into the photoproduct. This indicated the presence of a reverse (photoinduced or spontaneous) thiol → thione process fast enough to compete with the V → VI phototautomeric conversion.
image file: d4cp02799j-s2.tif
Scheme 2 Photogeneration of two tautomers of 1-methyl-2-thiouracil, produced upon UV (λ = 305 nm) irradiation of the most stable oxo-thione form and the spontaneous thiol → thione tunneling transformation converting the photoproduced oxo-thiol form VI into the lowest energy oxo-thione tautomer V.

The UV-irradiated Ar matrix, containing monomers of 1-methyl-2-thiouracil, was subsequently kept in the dark and at 3.5 K. During this period, hydrogen-atom tunneling led to the transformation of the oxo-thiol form VI into the lowest energy oxo-thione tautomer V (Scheme 2, Fig. 3 and Fig. S4, ESI†). This process was much faster than the analogous thiol → thione tunneling observed for 2-thiouracil. The time constant of the VI → V hydrogen-atom tunneling in 1-methyl-2-thiouracil was as small as 1.6 h (Table 1 and Fig. S6, ESI†). The form produced in this spontaneous process was easily identified by comparison of the growing IR spectrum with the spectrum recorded directly after deposition of the matrix (Fig. 3b and c). The identification of the form being consumed when the matrix was kept in the dark is also very reliable. The decreasing experimental IR spectrum (Fig. 3b) is reproduced very well by the spectrum theoretically predicted for the form VI (Fig. 3a and Table S2, ESI†). Other small bands due to the photoproduced hydroxy-thione form VII did not change their intensities during the period when the matrix was kept in the dark. This observation correlates well with the very high barrier (135 kJ mol−1) theoretically predicted for the VII → V, hydroxy → oxo spontaneous tautomerization.


image file: d4cp02799j-f3.tif
Fig. 3 Spectral effects of hydrogen-atom tunneling that converted the photoproduced oxo-thiol form VI of 1-methyl-2-thiouracil into the lowest-energy oxo-thione tautomer V: (a) theoretical spectrum calculated for the oxo-thiol form VI of the compound, (b) difference spectrum obtained by subtracting the spectrum recorded after UV (λ = 305 nm) irradiation of the matrix from the spectrum recorded after the next 16 hours when the Ar matrix was kept at 3.5 K and in the dark, and (c) infrared spectrum of monomeric 1-methyl-2-thiouracil recorded after deposition of the Ar matrix.

3.4 Hydrogen-atom tunneling in matrix-isolated molecules of 6-aza-2-thiouracil

Monomers of 6-aza-2-thiouracil were trapped in an Ar matrix. The IR spectrum of the matrix-isolated compound was the same as that previously reported.27 Analysis of this spectrum indicated that oxo-thione tautomer VIII is the only form of 6-aza-2-thiouracil populated in the Ar matrix. In the next step, matrix-isolated monomers of 6-aza-2-thiouracil were irradiated with UV (λ = 305 nm) light. After prolonged UV irradiation nearly all of the initial oxo-thione tautomers VIII were converted to the oxo-thiol form X (see Scheme 3, Fig. 4b, Fig. S7, S8, and Table S3, ESI†). The photogeneration of a very small amount of the hydroxy-thiol tautomer XI (Scheme 3) was revealed by the appearance of a weak νOH band at 3554/3552 cm−1 in the infrared spectrum recorded after UV (λ = 305 nm) irradiation (Fig. S8, ESI†). The very weak bands at 3398, 1730 and 1490 cm−1, which appeared only at the early stages of UV irradiation, can be attributed to the photoproduced oxo-thiol form IX (Scheme 3). On the whole, the photochemical behavior of the matrix-isolated monomers of 6-aza-2-thiouracil is similar to that of 2-thiouracil and 6-aza-2-thiothymine.25
image file: d4cp02799j-s3.tif
Scheme 3 Photogeneration of three tautomers of 6-aza-2-thiouracil, produced upon UV (λ = 305 nm) irradiation of the most stable oxo-thione form VIII and the spontaneous thiol → thione tunneling transformation converting the photoproduced form X into the lowest energy form VIII.

image file: d4cp02799j-f4.tif
Fig. 4 High-frequency fragment of the infrared spectra of 6-aza-2-thiouracil monomers isolated in a low-temperature Ar matrix: (a) after deposition of the matrix, (b) after 6 hours of exposure to UV (λ = 305 nm) light, (c) after subsequent 96 hours when the matrix was kept at 3.5 K and in the dark, and (d) difference spectrum obtained by subtraction of spectrum “b” from spectrum “c”.

The UV-irradiated matrix, containing monomers of 6-aza-2-thiouracil, was subsequently kept in the dark and at 3.5 K. During this period, a spontaneous thiol → thione transformation proceeded at a very low rate (Fig. 4b–d, Fig. 5 and Scheme 3). The progress of the hydrogen-atom tunneling conversion, transforming the oxo-thiol form X into the most stable oxo-thione form VIII, was significantly slower (with a time constant of 1160 h) than the progress of the analogous process observed for 2-thiouracil (Fig. 6). This can be related to the higher barrier (99 kJ mol−1, Table 1) predicted at the MP2 level for the spontaneous X → VIII transformation in monomeric 6-aza-2-thiouracil.


image file: d4cp02799j-f5.tif
Fig. 5 Spectral effects of hydrogen-atom tunneling that converted the photoproduced oxo-thiol form X of 6-aza-2-thiouracil to the most stable oxo-thione form VIII: (a) theoretical spectrum calculated for the oxo-thiol form X of the compound, (b) difference spectrum obtained by subtracting the spectrum recorded after UV (λ = 305 nm) irradiation of the matrix from the spectrum recorded after the next 96 hours when the Ar matrix was kept at 3.5 K and in the dark, and (c) infrared spectrum of monomeric 6-aza-2-thiouracil recorded after deposition of the Ar matrix.

image file: d4cp02799j-f6.tif
Fig. 6 Comparison of the progress of the thiol → thione hydrogen-atom tunneling in 2-thiouracil (red, squares), 1-methyl-2-thiouracil (blue, asterisks) and 6-aza-2-thiouracil (black, triangles). The process was observed for monomers of the compounds isolated in an Ar matrix and kept at 3.5 K in the dark. For 2-thiouracil, 1-methyl-2-thiouracil and 6-aza-2-thiouracil, the progress of the thiol → thione conversion was measured as an intensity decrease of the band at 1243 cm−1 (due to form III), the band at 1692 cm−1 (due to form VI) and the band at 1545 cm−1 (due to form X), respectively. The solid black, red, and blue lines represent functions I(t) = I(t0) exp (−t/τ) fit to the experimental points: I(t1)/I(t0), I(t2)/I(t0), I(t3)/I(t0) ….

4 Conclusions

The experiments carried out within the current work demonstrated that, for the majority of the tested heterocyclic compounds without intramolecular hydrogen bonds, spontaneous unimolecular hydrogen-atom transfer does not occur. The reason for this is the height of the barriers (usually greater than 100 kJ mol−1), which make the probability of intramolecular hydrogen-atom tunneling extremely low. Nevertheless, in the present study, we found exceptional cases of thiol → thione transformations in 2-thiouracil and 1-methyl-2-tiouracil, where the theoretically predicted barriers for the thiol → thione tautomerization are somewhat lower, 93 and 88 kJ mol−1, respectively.

For these two compounds, the spontaneous conversions of the higher-energy oxo-thiol tautomer into the most stable oxo-thione form occurred and were experimentally observed. In 6-aza-2-thiouracil, the thiol → thione hydrogen-atom tunneling also occurred, but proceeded at an extremely low rate. Despite the structural similarity and not very large differences in the theoretical barrier heights, the spontaneous tautomerizations in 1-methyl-2-thiouracil, 2-thiouracil and 6-aza-2-thiouracil proceeded at very different rates characterized by time constants of 1.6, 76 and 1160 hours, respectively (see Fig. 6). This demonstrates how a sensitive function of the barrier parameters is the probability of hydrogen-atom tunneling.28

Data availability

The spectral data and the results of calculations are available upon request from mjnow@ifpan.edu.pl.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We gratefully acknowledge the support from the Interdisciplinary Centre for Mathematical and Computational Modelling of the University of Warsaw, Grant G78-16. The authors acknowledge the help of Dr Anna Luchowska in experiments with 6-aza-2-thiouracil.

References

  1. P. R. Schreiner, Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm, J. Am. Chem. Soc., 2017, 139, 15276–15283 CrossRef CAS .
  2. P. R. Schreiner, Quantum Mechanical Tunneling Is Essential to Understanding Chemical Reactivity, Trends Chem., 2020, 2, 980–989 CrossRef CAS .
  3. J. Meisner and J. Kästner, Atom Tunneling in Chemistry, Angew. Chem., Int. Ed., 2016, 55, 5400–5413 CrossRef CAS .
  4. C. M. Nunes, I. Reva and R. Fausto, Direct Observation of Tunnelling Reactions by Matrix Isolation Spectroscopy, in Theoretical and Computational Chemistry, Series 18, Tunnelling in Molecules: Nuclear Quantum Effects from Bio to Physical Chemistry, ed. J. Kästner and S. Kozuch, The Royal Society of Chemistry, 2021, ch. 1 Search PubMed .
  5. R. Fausto, G. O. Ildiz and C. M. Nunes, IR-Induced and Tunneling Reactions in Cryogenic Matrices: the (Incomplete) Story of a Successful Endeavor, Chem. Soc. Rev., 2022, 51, 2853–2872 RSC .
  6. I. Reva, M. J. Nowak, L. Lapinski and R. Fausto, Spontaneous Tunneling and Near-Infrared-Induced Interconversion between the Amino-Hydroxy Conformers of Cytosine, J. Chem. Phys., 2012, 136, 064511 CrossRef .
  7. B. Bernhardt, F. Dressler, A. K. Eckhardt, J. Becker and P. R. Schreiner, Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide, Chem. – Eur. J., 2021, 27, 6732–6739 CrossRef CAS .
  8. S. Gobi, C. Nunes, I. Reva, G. Tarczay and R. Fausto, S-H Rotamerization via Tunneling in a Thiol Form of Thioacetamide, Phys. Chem. Chem. Phys., 2019, 21, 17063–17071 RSC .
  9. S. Henkel, Y. Huynh, P. Neuhaus, M. Winkler and W. Sander, Tunneling Rearrangement of 1-Azulenylcarbene, J. Am. Chem. Soc., 2012, 134, 13204–13207 CrossRef CAS .
  10. A. K. Eckhardt, F. R. Erb and P. R. Schreiner, Conformer-Specific [1,2]H-Tunnelling in Captodatively-Stabilized Cyanohydroxycarbene (NC–C–OH), Chem. Sci., 2019, 10, 802–808 RSC .
  11. P. R. Schreiner, H. P. Reisenauer, D. Ley, D. Gerbig, C. H. Wu and W. D. Allen, Methylhydroxycarbene: Tunneling Control of a Chemical Reaction, Science, 2011, 332, 1300–1303 CrossRef CAS PubMed .
  12. J. P. L. Roque, C. M. Nunes, L. P. Viegas, N. A. M. Pereira, T. M. V. D. Pinho e Melo, P. R. Schreiner and R. Fausto, Switching on H-Tunneling through Conformational Control, J. Am. Chem. Soc., 2021, 143, 8266–8271 CrossRef CAS PubMed .
  13. J. P. L. Roque, C. M. Nunes, P. R. Schreiner and R. Fausto, Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects, Chem. – Eur. J., 2024, 30, e202401323 CrossRef CAS PubMed .
  14. H. Rostkowska, L. Lapinski, A. Khvorostov and M. J. Nowak, Proton-Transfer Processes in Thiourea: UV Induced Thione → Thiol Reaction and Ground State Thiol → Thione Tunneling, J. Phys. Chem. A, 2003, 107, 6373–6380 CrossRef CAS .
  15. H. Rostkowska, L. Lapinski and M. J. Nowak, Hydrogen-Atom Tunneling Through a Very High Barrier; Spontaneous Thiol - Thione Conversion in Thiourea Isolated in Low-Temperature Ar, Ne, H2 and D2 Matrices, Phys. Chem. Chem. Phys., 2018, 20, 13994–14002 RSC .
  16. L. Lapinski, H. Rostkowska, A. Khvorostov, M. Yaman, R. Fausto and M. J. Nowak, Double-Proton-Transfer Processes in Dithiooxamide: UV-Induced Dithione → Dithiol Reaction and Ground-State Dithiol → Dithione Tunneling, J. Phys. Chem. A, 2004, 108, 5551–5558 CrossRef CAS .
  17. H. Rostkowska, L. Lapinski, A. Khvorostov and M. J. Nowak, Proton Transfer Processes in Selenourea: UV-Induced Selenone → Selenol Photoreaction and Ground State Selenol → Selenone Proton Tunneling, Chem. Phys., 2004, 298, 223–232 CrossRef CAS .
  18. H. Rostkowska, L. Lapinski and M. J. Nowak, Hydrogen-Atom Tunneling in Small Thioamides: N-Methylthiourea, Thiobenzamide and 2-Cyanothioacetamide, Spectrochim. Acta. A, 2024, 313, 124139 CrossRef CAS PubMed .
  19. A. D. Becke, Density-functional Exchange-energy Approximation with Correct Asymptotic Behavior, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 3098–3100 CrossRef CAS .
  20. C. T. Lee, W. T. Yang and R. G. Parr, Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789 CrossRef CAS .
  21. S. H. Vosko, L. Wilk and M. Nusair, Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: a Critical Analysis, Can. J. Phys., 1980, 58, 1200–1211 CrossRef CAS .
  22. C. Møller and M. S. Plesset, Note on an Approximation Treatment for Many-electron Systems, Phys. Rev., 1934, 46, 618–622 CrossRef .
  23. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci and G. A. Petersson, et al., Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, 2013 Search PubMed .
  24. L. Lapinski, H. Rostkowska and M. J. Nowak, Distinct Class of Photoinduced Hydrogen-Atom-Transfer Processes: Phototautomerizations in Molecules with no Intramolecular Hydrogen Bond in the Structure, Int. Rev. Phys. Chem., 2022, 41, 1–47 Search PubMed .
  25. A. Khvorostov, L. Lapinski, H. Rostkowska and M. J. Nowak, UV-Induced Generation of Rare Tautomers of 2-Thiouracil: A Matrix-Isolation Study, J. Phys. Chem. A, 2005, 109, 7700–7707 CrossRef CAS .
  26. H. Rostkowska, K. Szczepaniak, M. J. Nowak, J. Leszczynski, K. KuBulat and W. B. Person, Tautomerism and Infrared Spectra of Thiouracils. Matrix Isolation and ab Initio Studies, J. Am. Chem. Soc., 1990, 112, 2147–2160 CrossRef CAS .
  27. L. Lapinski, D. Prusinowska, M. J. Nowak, M. Bretner, K. Felczak, G. Maes and L. Adamowicz, Infrared Spectra of 6-Azathiouracils: An Experimental Matrix Isolation and Theoretical Ab Initio SCF/6-311G** Study, Spectrochim. Acta, Part A, 1996, 52, 645–659 CrossRef .
  28. G. Qiu and P. R. Schreiner, The Intrinsic Barrier Width and its Role in Chemical Reactivity, ACS Cent. Sci., 2023, 9, 2129–2137 CrossRef CAS .

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp02799j
This journal is © the Owner Societies 2024