Theoretical studies on geometry, solvent effect, and photochromic mechanism of two bis-heterocyclic compounds containing pyrazolone ring

Theoretical Studies on Geometry,Solvent Effect, and PhotochromicMechanism of Two Bis-HeterocyclicCompounds Containing Pyrazolone Ring

ANJIE LIU, DONGLING WU, DIANZENG JIA, LANG LIUInstitute of Applied Chemistry, Xinjiang University, Urumqi 830046, People’s Republic of China

Received 2 March 2009; accepted 27 March 2009Published online 29 September 2009 in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/qua.22280

ABSTRACT: Two bis-heterocyclic compounds containing pyrazolone ring, 1-phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one and 1-phenyl-3-methyl-4-(6-hydro-4-methylamino-5-sulfo-2,3-pyrazine)-pyrazole-5-one, are investigatedto gain a deeper insight into their geometries and photochromic mechanism byapplying density functional theory. The solvent effects are simulated using thepolarizable continuum model of the self-consistent reaction field theory. Bader’s atom-in-molecule theory is used to investigate the nature of hydrogen bonds. The data ofenergy, dipole moments, and the condensed Fukui functions have been calculated toassess the stability and reactivity of the title compounds. © 2009 Wiley Periodicals, Inc.Int J Quantum Chem 110: 1360–1367, 2010

Key words: bis-heterocyclic; DFT; solvent effects; AIM; Fukui function

Correspondence to: D. Jia; e-mail: [email protected] grant sponsor: National Natural Science Foundation

of China.Contract grant number: 20762010.Contract grant sponsor: Scientific Research Program of the

Higher Education Institution of Xinjiang.Contract grant number: XJEDU2006I04.Contract grant sponsor: Young Scholar Science Foundation of

Xinjiang University.Contract grant number: QN070116.

International Journal of Quantum Chemistry, Vol 110, 1360–1367 (2010)© 2009 Wiley Periodicals, Inc.

Introduction

P hotochromic compounds have attracted con-siderable attention in recent years because of

their potential applications, such as optical diskmemory, switching devices, and liquid crystalalignment [1, 2]. Many studies have focused ondeveloping new photochromic systems [3–7],which give the possibilities to probe the relation-ship between geometry and photochromic proper-ties and explore the photochromic mechanism.

In our laboratory, many novel photochromiccompounds containing pyrazolone ring [6–10] havebeen synthesized and studied. On the basis of thecrystal structures of photochromic compounds, it isproposed that the photochromic phenomenon wasdue to intramolecular (intra-PT) or intermolecularproton transfer (inter-PT) through hydrogen bonds(H-bonds). However, experimental methods aresometimes insufficient for further achievements onthe mechanism of photochromism, and thus theo-retical studies are required. Our previous theoreti-cal studies [11–13] have done many works on dis-cussing the intra-PT or inter-PT mechanism of thecompounds with the thiosemicarbazone structurein the gas phase and in solution.

To gain a deeper insight into the photochromicmechanism and the solvent effects on the geometriesand chemical reactivity, two bis-heterocyclic com-pounds, 1-phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one(PMCP-TSC) [14]and 1-phenyl-3-methyl-4-(6-hydro-4-methylamino-5-sulfo-2,3-pyrazine)-pyrazole-5-one (PMCP-MTSC)[15] have been studied. Previous experimental results[14] indicate that PMCP-TSC undergoes photo-chromism in solution, however, the phenomenon hasnot been found on the latter compound. It is proposedthat the photochromic mechanism of PMCP-TSC is anintra-PT from the enol form to keto form (Fig. 1). Inthis article, we have analyzed the molecular structure,the nature of H-bond concerned with intra-PT pro-cess, and the stability and reactivity of the title com-pounds in different solvents. This work is helpful toobtain a deeper understanding of the photochromicmechanism in solution and will make a good foun-dation for further research.

Computation Details

All calculations of the title compounds were per-formed with the Gaussian 03W program [16]. The

B3LYP/6-311�G (2d, p) level of theory was used inthis article. This level of theory has been justified inour previous work [13]. Vibrational analysis con-firmed that the local minima on the potential en-ergy surface had all positive frequencies. Solventeffects were studied using the self-consistent reac-tion field method and the polarizable continuummodel [17]. The natural bond orbital (NBO) analysiswas performed by means of the NBO 3.1 program[18] within the Gaussian 03W package. The relativereactivity of different sites in molecules was inves-tigated by the condensed Fukui functions [19].

Results and Discussion

THE SOLVENT EFFECTS ON THESTRUCTURES

The structures of the tautomers for the title com-pounds are presented in Figure 1. To further studythe reactive mechanism of the title compounds, theketo and constructed enol forms are all optimizedby using B3LYP/6-311�G(2d,p) level. The struc-tural parameters of keto form are in good agree-ment with the X-ray data. In this part, structuraldifference in different solvents including water,methanol, tetrahydrofuran (THF), and carbon tetra-chloride (CCl4) are investigated in detail.

For the keto and enol forms of PMCP-TSC, boththe S1 and C8 atoms have great deviation with theplane I [N(4)-N(5)-C(9)-C(10)]. Furthermore, it isfound that the angles between plane I and pyra-zolone ring (plane II) are almost 180°, but the anglesbetween plane II and benzene ring (plane III) varyfrom 141° in enol form to 178° in keto form. Previ-ous experimental studies have validated that the

FIGURE 1. The structures of tautomers for the titlecompounds.

TWO BIS-HETEROCYCLIC COMPOUNDS CONTAINING PYRAZOLONE RING

VOL. 110, NO. 7 DOI 10.1002/qua INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 1361

enol form can exhibit photochromism but the ketoform can not. Cohen and coworkers [20–22] hasproposed that the Schiff base compounds exhibit-ing thermochromism are planar and the com-pounds exhibiting photochromism are nonplanar.To get a deeper insight into the relationship be-tween the coplanarity of structures and photochro-mic properties, the angles between plane I and II aswell as plane II and III for the title compounds indifferent solvents are calculated and listed in TableI. It is worth noting that the calculated results showsmall differences in the angles between plane I andII for enol and keto forms in these solvents, whichindicates that the solvent can hardly affect the co-planarity between plane I and II. In addition, theangles between plane II and III for enol form growwith the decrease of the solvent polarity. Previousexperiments show that the polarity increase of thesolvent favors the phototautomerization. It meansthat the noncoplanarity between II and phenyl isbeneficial to photochromic property. For PMCP-

MTSC, the trend on geometrical difference in vari-ous solvents is similar to that of PMCP-TSC.

In this part, the relationship between angles ofplane II and III for enol form and dielectric constantof solvent is established and shown in Figure 2.Four different solvents (water, methanol, THF, andCCl4) are chosen, and their dielectric constants are78.39, 32.63, 7.58, and 2.23, respectively. The corre-lation is reverse, that is, an increase in dielectricconstant corresponds to a decrease in angles. ForPMCP-TSC and PMCP-MTSC, attempts to find thelinear correlation only yield poor values of coeffi-cient (R � 0.838, R � 0.775), but the trend is ex-pected.

Previous studies [14] have proposed that thephotochromic mechanism of PMCP-TSC in metha-nol is intra-PT between OOH��N/O��HON tauto-meric equilibrium (Fig. 1). It was also found thatOOH��N and O��HON should be the strongestH-bonds, which may facilitate the proton transferreaction. In this part, to investigate the solvent ef-

TABLE I ______________________________________________________________________________________________The angles (°) between different planes for the title compounds.

PMCP-TSC PMCP-MTSC

H2O CH3OH THF CCl4 H2O CH3OH THF CCl4

Enol 179.806a 179.990 179.828 179.501 179.840 179.946 179.938 179.326139.288b 141.528 146.324 153.721 140.674 142.144 146.036 156.330

Keto 178.137a 177.739 177.399 176.666 178.214 178.179 177.417 176.984179.098b 179.934 179.430 179.802 179.311 179.607 179.637 179.883

a The angles between plane I and II.b The angles between plane II and III.

FIGURE 2. The correlations between angles (plane II and III) and dielectric constant of enol form for PMCP-TSC (a)and PMCP-MTSC (b).

LIU ET AL.

1362 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY DOI 10.1002/qua VOL. 110, NO. 7

fect on the nature of the abovementioned H-bonds,two theoretical techniques are applied. One of themis atom-in-molecule (AIM) theory [23–25], whichhas been successfully used to describe interatomicinteractions. In this theory, the value of electrondensity �(r) and the sign of the Laplacian of theelectron density �2�(r) at the H-bond critical points(HBCP) can characterize the nature of H-bond. Theother is the NBO theory [26]. In terms of the theory,H-bond (XOH��Y) contact can be attributed to theelectronic delocalization from the filled lone pair ofthe electron donor (nY) into the unfilled antibond-ing of the electron acceptor (nY 3 �XOH

*). Thesecond-order perturbation energy E(2) reflects theattractive interaction in H-bond and can be used tocharacterize the strength of H-bond. Electron den-sities and their Laplacians at HBCPs as well asstabilization energy E(2) of OOH��N/O��HON H-bonds are collected in Table II. It shows that allH��Y bonds have large �(r) (ranging from 0.0250 to0.0357 au) and positive �2�(r) values (ranging from0.1378 to 0.2539 au), which indicates that these H-bonds are very strong and will be the point ofreaction. Furthermore, the AIM analysis for theenol form of PMCP-TSC reveals that the strength ofH-bond grows with the decrease of the solventpolarity. The reason would be that the structure ofnonplanar will induce the length of H��Y to be

large. The same phenomenon occurs in PMCP-MTSC.

The linear correlations between the H-bondlength and topological parameters have been foundin our previous studies [12]. In this article, to ana-lyze the relationship between H-bond topologicalparameter and stabilization energy for the studiedsystems, the correlations between �(r)/�2�(r) andE(2) are established and shown in Figures 3 and 4.Good linear correlation (0.9503, 0.9992) between�(r)/�2�(r) and E(2) indicates that both H-bond to-pological parameters and stabilization energy cancharacterize the H-bond strength of the title com-pounds.

SOLVENT EFFECTS ON THE TAUTOMERSSTABILITY

The results from Table III show that the ketoform of PMCP-TSC is more stable than the enolform in all solvents, and their stabilities grow withthe increase of the solvent polarity. The relativeenergy for enol form to keto form is 8.808 kcal/molin water, 8.705 kcal/mol in methanol, 8.312 kcal/mol in THF, and 7.458 kcal/mol in CCl4, respec-tively. It is clear that the value of �E also growswith the increase of the solvent polarity, whichindicates that the reaction from enol to keto in

TABLE II _____________________________________________________________________________________________Electron densities (au) and their Laplacians (au) at HBCPs as well as stabilization energy (kcal/mol) ofOOH��N/O��HON H-bonds.

PMCP-TSC Solvent E(2) �XH �2�XH �HY �2�HY

OOH��N H2O 30.92 0.3064 �2.2454 0.0344 0.2400CH3OH 32.51 0.3043 �2.2310 0.0351 0.2478THF 32.58 0.3046 �2.2337 0.0352 0.2487CCl4 32.55 0.3050 �2.2384 0.0354 0.2497

O��HON H2O 10.23 0.3313 �1.8358 0.0250 0.1381CH3OH 10.64 0.3316 �1.8360 0.0254 0.1407THF 11.51 0.3310 �1.8374 0.0259 0.1457CCl4 12.37 0.3295 �1.8330 0.0270 0.1553

PMCP-MTSC

OOH��N H2O 33.99 0.3011 �2.2075 0.0357 0.2539CH3OH 33.18 0.3029 �2.2211 0.0354 0.2503THF 34.79 0.3006 �2.2050 0.0362 0.2584CCl4 34.23 0.3018 �2.2166 0.0361 0.2572

O��HON H2O 10.24 0.3316 �1.8348 0.0250 0.1378CH3OH 10.39 0.3320 �1.8372 0.0252 0.1392THF 11.55 0.3307 �1.8343 0.0263 0.1485CCl4 12.15 0.3301 �1.8366 0.0268 0.1524



ground state is more exothermic in greater polarityof the solvent. In addition, the dipole moments ofketo are larger than those of enol in all solvents, andthe dipole moments of keto and enol forms increasewith the growing of the solvent polarity, which is inagreement with the trend of �E. As for PMCP-MTSC, the trend of stability is the same as that ofPMCP-TSC.

It is well known that the free energy of solvation,�Gsolv, is the work needed to transfer moleculefrom gas phase into solution, which plays a signif-icant role in the understanding of the chemical be-havior of molecules in condensed phase. Calculated�Gsolv of the title compounds are listed in Table III.For PMCP-TSC, the date show that the �Gsolv valueof keto form is lower than that of enol form. Fur-thermore, it can be found that the �Gsolv valuedecreases with the increase of the organic solventpolarity, and thus, the solvation of PMCP-TSC in-creases with the polarity of the solvent. In addition,although the polarity of water is stronger than thatof methanol, the value of �GCH3OH is more negativethan that of H2O, which means that the solvation of

PMCP-TSC in methanol is better than in water.Moreover, the �Gsolv value of PMCP-TSC in differ-ent solvents is lower than that of PMCP-MTSC. Itmeans that the methyl group bonded to N3 atomshall be a hindrance for PMCP-MTSC to form H-bond with solvent.

REACTIVITY IN DIFFERENT SOLVENTS

The atom-condensed Fukui function (fk�) (� � �,

0, �) proposed by Yang and Mortier [19] has beenfrequently used to describe the site reactivity or siteselectivity [27–30]. In this part, fk

� is calculated anddiscussed to study the reactivity of the title com-pounds in different solvents. Table IV quotes thevalues of fk

� to the main atoms that define thereactive region of the title compounds in differentsolvents. Different fk

� values to the given atom indifferent solvents indicate that the solvent can affectthe site reactivity.

For enol form of PMCP-TSC in different solvents,the N6 atom has the lowest fk

� value which indi-cates that this site would not be the favorable one

FIGURE 3. Correlations between �(r)/�2�(r) and E(2) of enol and keto forms for PMCP-TSC. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com.]

LIU ET AL.


FIGURE 4. Correlations between �(r)/�2�(r) and E(2) of enol and keto forms for PMCP-MTSC. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com.]

TABLE III ____________________________________________________________________________________________Electronic energies (E) (a.u.), free energies of solvation (�Gsolv) (kcal/mol), dipole moments (�) (Debye), andrelative energies for enol form to keto form (�E) (kcal/mol).

Solvent

Enol Keto

�EE �Gsolv � E �Gsolv �

PMCP-TSC

H2O �1250.646344 �3.19 5.374 �1250.660380 �5.96 10.963 �8.808CH3OH �1250.645195 �9.57 5.370 �1250.659068 �12.11 10.768 �8.705THF �1250.640572 �3.48 5.073 �1250.653819 �5.63 10.051 �8.312CCl4 �1250.631517 1.71 4.458 �1250.643402 0.53 8.548 �7.458

PMCP-MTSC

H2O �1289.961506 0.71 6.013 �1289.976275 �2.16 11.574 �9.268CH3OH �1289.960365 �6.58 5.905 �1289.974948 �9.21 11.409 �9.151THF �1289.955831 �0.18 5.596 �1289.969732 �2.39 10.522 �8.723CCl4 �1289.947547 4.2 4.921 �1289.960126 2.95 9.087 �7.893



for electrophilic attack. The S1, N3, N4, N5, and N7atoms possess large Fukui function. Because thereis only one O2OH��N5 interaction in the isolatedmolecule, N5 is the possible site for reactivity. Inaddition, it is found that the values of S1 and N6decrease with the growing of the solvent polarity.On the contrary, the values of other atoms espe-cially N5 grow with the growing of the solventpolarity, which indicates that the growing of sol-vent polarity will enhance the reactivity of theseatomic sites. Considering the keto form of PMCP-TSC, it is obvious that the O2 atom has the highestfk� value and should be the most favorable atomic

site for electrophilic attack. Furthermore, with thegrowing of solvent polarity, the possibility of elec-trophilic attack for O2 atom increases. As forPMCP-MTSC, it has the similar reactivity. The onlydifference is that the fk

� values of PMCP-MTSC arelower than that of PMCP-TSC.

Conclusions

The solvent effects on the structures of the twocompounds have been investigated. The experi-mental and computational results indicate that theangles between pyrazolone-ring and benzene-ringhave a close relationship with the photochromicproperty. The linear correlation between the angles

and the dielectric constant of solvent is poor but thetrend is expected. In addition, the strength of H-bond has been found to increase with the decreaseof the solvent polarity, and good correlation be-tween H-bond topological parameter and stabiliza-tion energy has been established.

The keto form of the title compounds is morestable than the enol form in all appointed solvents,and the stability of enol form and keto form growswith the increase of the solvent polarity. In addi-tion, the data of �Gsolv value shows that the solva-tion of the title compounds increases with thegrowing of the organic solvent polarity, and thesolvation in methanol is better than in water thoughthe polarity of water is stronger than that of meth-anol.

The calculated atom-condensed Fukui functionpredicts that the most probable reactive sites forelectrophilic attack are N5 atom in enol form andO2 atom in keto form, and the reactivity increaseswith the increase of the solvent polarity.

References

1. Irie, M. Chem Rev 2000, 100, 1685.

2. Kurihara, S.; Ikedu, T.; Tazuke, S. J. J Appl Phys 1988, 63,1791.

TABLE IV ____________________________________________________________________________________________Condensed Fukui functions fk

� at selected atoms.

PMCP-TSC Solvent S1 O2 N3 N4 N5 N6 N7

Enol H2O 0.088 0.065 0.095 0.120 0.106 0.017 0.111CH3OH 0.088 0.065 0.093 0.116 0.105 0.017 0.114THF 0.089 0.063 0.089 0.105 0.104 0.019 0.114CCl4 0.089 0.054 0.079 0.080 0.101 0.029 0.104

Keto H2O 0.057 0.170 0.045 0.052 0.086 0.060 0.139CH3OH 0.057 0.169 0.045 0.049 0.085 0.062 0.135THF 0.057 0.166 0.041 0.037 0.077 0.074 0.118CCl4 0.053 0.150 0.035 0.021 0.065 0.091 0.081

PMCP-MTSC

Enol H2O 0.081 0.060 0.114 0.132 0.094 0.017 0.098CH3OH 0.081 0.060 0.112 0.129 0.093 0.017 0.100THF 0.082 0.061 0.107 0.117 0.091 0.017 0.104CCl4 0.078 0.047 0.095 0.092 0.092 0.031 0.087

Keto H2O 0.058 0.157 0.060 0.065 0.081 0.049 0.146CH3OH 0.058 0.157 0.060 0.064 0.080 0.051 0.143THF 0.059 0.155 0.053 0.048 0.073 0.061 0.129CCl4 0.055 0.151 0.040 0.026 0.071 0.081 0.090

LIU ET AL.


3. Pozzo, J. L.; Lokshin, V.; Samat, A.; Guglielmetti, R.; Dubest,R.; Aubard, J. J Photochem Photobiol A Chem 1998, 114, 185.

4. Tsujioka, T.; Kume, M.; Irie, M. J Photochem Photobiol AChem 1997, 104, 203.

5. Ying, L.; Xie, M. G. Funct Mater 1998, 29, 113 (in Chinese).6. Tang, X. C.; Jia, D. Z.; Liang, K.; Zhang, X.G.; Xia, X.; Zhou,

Z. Y. J Photochem Photobiol A Chem 2000, 134, 23.7. Liu, L.; Jia, D. Z.; Ji, Y. L.; Yu, K. B. J Photochem Photobiol A

Chem 2003, 154, 117.8. Peng, B. H.; Liu, G. F.; Liu, L.; Jia, D. Z. Tetrahedron 2005, 61,

5926.9. Zhang, T.; Liu, G. F.; Liu, L.; Jia, D. Z.; Zhang, L. Chem Phys

Lett 2006, 427, 443.10. Guo, J. X.; Liu, L.; Liu, G. F.; Jia, D. Z.; Xie, X. L. Org Lett

2007, 20, 3989.11. Guo, Y.; Liu, G. F.; Liu, L.; Jia, D. Z. J Mol Struct (Theochem)

2004, 712, 223.12. Wu, D. L.; Liu, L.; Liu, G. F.; Jia, D. Z. J Mol Struct (Theo-

chem) 2007, 806, 197.13. Lin, H.; Wu, D. L.; Liu, L.; Jia, D. Z. J Mol Struct (Theochem)

2008, 850, 32.14. Liu, L.; Jia, D. Z.; Qiao, Y. M.; Yu, K. B. Acta Chim Sin 2002,

60, 493.15. Liu, L.; Liu, G. F.; Jia, D. Z.; Yu, K. B. Chin J Struct Chem

2003, 22, 203.16. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr.T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.;Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara,M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Naka-jima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.;Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jara-millo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;

Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghava-chari, K.; Foresman, J. B.; Ortz, J. V.; Cui, Q.; Baboul, A. G.;Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-combe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B. 04;Gaussian Inc.: Pittsburgh, PA, 2003.

17. Amovilli, C.; Barone, V.; Cammi, R.; Cances, E.; Cossi, M.;Mennucci, B.; Pomelli, C. S.; Tomasi, J. Adv Quantum Chem1998, 32, 227.

18. Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,J. E.; Weinhold, F. NBO Version 3.1; Theoretical ChemistryInstitute, University of Wisconsin: Madison, 1995.

19. Yang, W.; Mortier, W. J. J Am Chem Soc 1986, 108, 5708.20. Cohen, M. D.; Schmidt, G. M. J. J Phys Chem 1962, 66, 2442.21. Cohen, M. D.; Hirshberg, Y.; Schmidt, G. M. J.; Flavian, S.

J Chem Soc 1964, 2051.22. Cohen, M. D.; Flavian, S. J Chem Soc 1967, 334.23. Bader, R. F. W. Atoms in Molecules: A Quantum Theory;

Clarendon Press: Oxford, 1990.24. Bader, R. F. W.; Gillespie, R. J.; MacDougall, P. J. J Am Chem

Soc 1988, 110, 7329.25. Bader, R. F. W. Chem Rev 1991, 91, 893.26. Foster, J. P.; Weinhold, F. J Am Chem Soc 1980, 102, 7211.27. Padmanabhan, J.; Parthasarathi, R.; Sarkar, U.; Subramanian,

V.; Chattaraj, P. K. Chem Phys Lett 2004, 383, 122.28. Kolandaivel, P.; Praveena, G.; Selvarengan, P. J Chem Sci

2005, 117, 591.29. Kostova, I.; Trendafilova, N.; Momekov, G. J Inorg Biochem

2005, 99, 477.30. Wu, D. L.; Jia, D. Z.; Liu, L.; Liu, A. J. Int J Quantum Chem

2009, 109, 1341.



Documents

Theoretical studies on geometry, solvent effect, and photochromic mechanism of two bis-heterocyclic compounds containing pyrazolone ring