Molecular vibrations, electronic structure and conformation of diprotonated thiocarbohydrazide

Structure, 71 (1981) 51-59 Elsevier Scientific Publishing Company, Amsterdam -

Journal of Molecular

MOLECULAR FORMATION

S. MANOGARAN

Printed in The Netherlands

VIBRATIONS, ELECTRONIC STRUCTURE AND OF DIPROTONATED THIOCARBOHYDRAZIDE

CON-

and D. N. SATHYANARAYANA

Department of Inorganic 560012 (India)

and Physical

Chemistry.

Indian Institute

of Science,

Bangalore

K. VOLKA Department of Analytical (Czechoslovakia)

Chemistry,

Institute

of Chemical

Technology,

166

28 Prague 6

(Received 13 June 1980)

ABSTRACT The infrared spectra of diprotonated species of thiocarbohydrazide and its perdeuterated derivative have been examined in the crystalline state. A complete vibrational assignment with a full normal coordinate treatment based on a Urey-Bradley type intramolecular potential function supplemented with a valence force function for the out of plane and torsional modes is proposed and the origin of the amide II band splittings is explained. A CNDOIB study of diprotonated thiocarbohydrazide and its neutral molecule is undertaken and the changes in the molecular electronic structures and conformations consequent to protonation are determined and briefly discussed. The magnitude of the N-N’H, torsional barrier is estimated to be 21 kJ mol-’ (5.0 kcal mol-‘) whereas the barrier for the C-N group is found to be 92 kJ mol-’ (22.0 kcal mol-‘). INTRODUCTION

The structural and spectroscopic investigations of thiocarbohydrazide have received an impetus [l-8] in recent years owing to its varied pharmaceutical applications [ 9-121 and use as a good sulphur-nitrogen chelating agent [ 13-151. Further interest arises from the interaction of the hydrazine lone pair with the bonding electrons [ 161. Recently we reported an IR and Raman spectral study of thiocarbohydrazide (TCH), its metal complex, and selenocarbohydrazide [ 81. We have now examined the IR spectra of diprotonated thiocarbohydrazide (DPTCH, H3N+NHCSNHN+H3 2Cl-) and its perdeuterated species in the crystalline state and propose the assignment of all the fundamentals. An intramolecular potential field of the Urey-Bradley type supplemented with valence force functions for the out of plane modes is developed. Further, the magnitude of the barrier hindering the internal rotation about the N-N and C-N bonds has been calculated. The electronic structure and conformational stability of DPTCH with reference to the parent TCH molecule has been investigated by the self-consistent field, all valence, CND0/2 method. 0022-2860/81/0000--0000/$02.50

0 1981 Elsevier Scientific

Publishing

Company

52

X-ray dif&action studies have shown that there is a structural difference between DPTCH and its neutral molecule. It is found that TCH has a transcis conformation [4] while DPTCH has a cis-cis structure (I) [6]. The motivation for the present work was twofold. It seemed desirable to note firstly the variations in the assignments consequent upon change in conformation and protonation. Secondly, a study of the electronic structure of nitrogen protonated molecules is of interest in the discussion of many biological problems [ 173. It would be useful to obtain quantitative data on how much electron density is transferred from the neutral molecule to the proton on protonation, which atoms in the molecule contribute to the flow of charge, and how the 71electrons react to the transfer of sigma density. Also the present study of DPTCH and its neutral molecule should help to determine whether the conformational differences between the two species arise from electronic factors and/or are due to the complicated processes consequent upon crystal formation. A study of this kind may contribute to the understanding of similar structural differences in other molecules.

c/s-cis (I)

rmns -cis (II)

rruns-rruns

( III 1

EXPERIMENTAL

Thiocarbohydrazide was obtained as colourless crystals by the slow evaporation of a hydrochloric acid solution [6] of TCH at pH 0.5. The identity of the compound as TCH - 2HCl- 2H2 0 was established by chemical analysis. The deuterated compound was obtained by the exchange reaction with heavy water. The IR spectra were recorded on a Perkin-Elmer 325 and Carl Zeiss UR 10 IR spectrophotometers. No difference was observed between the spectrum of the compound in Nujol mull and that in a KBr pellet. The instruments were calibrated with the usual standards. THEORETICAL

TREATMENT

The calculations were done on the DEC 1090 system using programs written in FORTRAN IV. Normal

coordinate

analysis

The observed frequencies of DPTCH and its perdeuterated derivative were used in the calculations cast in terms of the GF matrix formulation. The molecular

conformation

is shown in Fig. 1 and the structural parameters

53

Fig. 1. Molecular structure of DPTCH.

used in the calculations were the X-ray diffraction values and are listed in Table 1. In the CzV point group model, the 36 fundamentals of DPTCH are classified amongst the symmetry species as 12A1 + 6A2 + 7B1 + 11B2 modes, where A2 and Bi modes are out of plane vibrations. The local symmetry coordinates were written in the usual form using internal valence coordinates and the torsional coordinates were defined according to Saito et al. [ 181. The initial values of the intramolecular force constants were transferred from the previously developed potential function for TCH, and for the -NH: portion from glycine [19]. A valence potential function was employed for the out of plane modes of the planar skeleton and for the -N+H, torsions. The force constants were refined through a least squares refinement algorithm by keeping the stretching constants fixed. The programs employed were similar to the normal coordinate package of Schachtschneider [ 201. Molecular

orbital calculations

The CND0/2 method in the standard form and parametrization suggested by Pople and SegaI [21] were employed. A full basis set of valence orbitals including the 3d orbitals of sulphur were considered. The geometry for DPTCH and TCH was based on the recent X-ray structure determinations (Table 1). The total molecular energy was calculated for the three more likely TABLE

1

Molecular Bond

parameters for DPTCH

and TCHa

Bond length

Angle

DPTCH

TCH

c-s

C-N

0.1663 0.1344

N-N

0.1410

N-H

0.0950

0.1724 0.1315(c) 0.1335(t) 0.1404(c) 0.1407(C) 1.0000

Bond angle DPTCH

TCH

NCN NCS

114.0 123.0

CN’H

120.0

HN’N

120.0

NNH HNH

109.47 109.47

117.5 124.0(c) 118.5(c) 122.0(c) 118.3(C) 115.6(c) 122.9(t) 109.47 109.47

aBond lengths in nm, angles in degrees. Abbreviations: group.

c and t denote cis and trans-NNCS

54 TABLE

2

Forceconstantsof DPTCHa Urey-Bradley type 4.40 4.87 4.85 5.70 3.50

KWH) K(N’H) K(NN) K(CN) K(CS) h(NH,)

-0.01

Valencetype

H(HNH) H(JXNN’) H(HN’N) H(HNC) H(NNC) H(NCN) H(NCS)

aUnih: K, N, Fare in N rad’).

0.417 0.367 0.200 0.285 0.263 0.586 0.274

F(HNH) F(HNN’) F(HN’N) F(HNC) F(NNC) F(NCN j F(NCS)

0.092 0.519 0.439 0.456 0.255 0.394 1.019

f(nCS) 0.105 f(nNH)0.050 f(&N) 0.095 f(~NN)0.045

cm-’ (= mdyn X-l) and h and f in lo-”

f(nNH, SN) (b,)

x N cm rad-

(= mdyn A

planar conformations of DPTCH shown above and for its neutral molecule assuming the geometrical parameters to remain the same during the internal rotation. The standard program of Pople and Beveridge was used [22]. RESULTS

0.025

f(XN, rNN) (b,) -0.030 f(rrNH, &N) (u2) -0.020

AND DISCUSSION

The final force constants for DPTCH are compiled in Table 2. The force constants obtained are of reasonable value and are similar to the appropriate ones for TCH. Spectral interpretations

The observed frequencies are compared with the calculated frequencies for DPTCH and its deuterated isotope in Table 3 where the calculated potential energy distributions, omitting for simplicity those of less than 15%, are shown. The IR spectrum of the sample (TCH - 2HCl- 2H2 0) has absorptions at 3430 (vs b), 3115 (s b) and 1608 (vs) cm-l corresponding to the vibrations of H,O molecules and these are not included in Table 3 (librational mode of the I-I20 molecules has been not detected due to overlap by the strong absorption bands of DPTCH in the range 600-800 cm-l). The vibrational frequencies for the -N’H3 group have similar values to those in related molecules, for instance, in hydrazinium hydrochloride [ 231, glycine [ 191 (zwitterion form) and the assignment for -N+Hj group vibrations will not be discussed. A brief discussion of’the features of the spectra of the thiourea skeleton of DPTCH will be given. The characteristic bands of interest are due to C=S and C-N stretching vibrations corresponding to thioamide I and III bands, respectively. The results of the coordinate analysis indicate a localized C=S stretching mode at 718 cm-* for DPTCH, in contrast to an extensively coupled [ 81 C=S mode for TCH. This behaviour is similar to the different nature of the C=S stretching

55 TABLE

3

Calculated and observed fundamentals (cm-l) Species

of DPTCH-d,

and d, and their assignmentsa

Descriptionb (PED, %)

DPTCH-d,

DPTCH-d,

talc.

Obs.

talc.

Obs.

a,

2234 2168 2083 1185 1083 993 1420 1377 868 679 391 176

2230 2160 2080 1185 1x21 1005 1423 1364 880 678 415 175

3119 2939 2905 1566 1537 1518 1421 1227 1133 717 421 196

3115 2940 2890 1569 1530 1520 1430 1227 1137 718 420 196

VNH( 99) u,NH,( 100) GH,(99) WH,(92) &NH,(W, pNH,(30) sNH( 73) rK!N( 48), uCS( 16) uNN( 58) pNH,(53), &NH,(lS) uCS(76), vCN(16) aNCN(44), &N(lS), aCNN(17) sCNN(70), sNCN( 28)

b:

2279 2168 2083 1563 1258 1089 1056 889 990 575 203

2230 2160 2080 1545 1250 1121 1060 905 980 567 213

3118 2939 2905 1614 1566 1521 1338 1198 1100 619 217

3115 2940 2890 1608 1569 1520 1335c 1196 1052 616 218

vNH( 99) vaNH,( 100) +NH,(99) &N(46), aNH(30) &.NH,(93) a,NH,(69), pNH,(21) aNH(GO), vCN( 33) pNH,( 53). 6,NH,( 22), YNN( 18) vNN(63), pNH,(17) 6 CS( 50), 6 CNN(40) aCNN(57), sCS(41)

6,

2165 1073 911 578 281 461 70

2160 1084 904 567 250 456 ni

2940 1508 1172 747 521 364 78

2940 1530 1183 756 526 370 ni

vNH,( 100) dNH,(73), pNH,(27) pNH,(72), s,NH,(27) rCN(39), rNN(31), rrNH(27) rNN( 53), n CS( 43) &S( 57), rNN(25), ;_CN( 18) XN(46), nNH(42)

2165 1072 912 515 218 237

2160 1121 930

2940 1507 1173 727 309 250

2940 1530 1183 718 310 250

vNH,( 100) aNH,(73), pNH,(27) pNH,(72), sNH,(27) rNN( 53), 7CN( 30), mNH( 18) rCN(41), 7NN(32), nNH( 27) srNH(70), rCN( 29)

226 234

aThe fundamentals of DPTCH-d, have been arranged so as to give as far as possible, approximately matching potential energy distribution (PED) stated For DPTCH-cf, ; ni, not investigated. by, stretching; 6, bending; P, rocking; 5~,out of plane bending and 7, torsion. CAverage of 1310 and 1352 cm-’ bands.

56

mode of thiosemicarbazide (TSC) and its metal chelates, where a similar conformational change takes place. The C=S stretching mode of TSC is highly coupled 1241 as against the localized C=S mode in the metal chelates [25]. The antisymmetic and symmetric C-N stretching frequencies of DPTCH at 1608 and 1430 cm-‘, respectively, compare reasonably with those of sym N,N’dimethyl thiourea (DMTU) [26] at 1560 and 1355 cm-l respectively. The assignments for other frequencies are compatible with the general spectral features of TCH [ 81, DMTU [ 261 and thiourea [ 271. The IR spectrum of DPTCH displays two absorptions of nearly equal intensity at 1310 and 1352 cm-l which disappear on Ndeuteration. They arise certainly from the antisymmetric NH bonding (amide II band) vibration, being split on account of intermolecular forces associated with dipoledipole interactions or weak intermolecular hydrogen bonds. Similar baud splittings in the Raman spectrum have been noticed for bans-trans dipropionamide [28]. The compound TCH - 2HCl- 2H2 0 crystallizes in the monoclinic system, space group C2/c with four molecules in the unit cell [6] . As seen from the correlation diagram for the crystalline DPTCH shown in Table 4, the a2 species bands which are IR forbidden in the tiee moleciz!e activate in the crystal as a consequence of the lowering of symmetry in the crystal. Molecular

conformation

The insolubility of the compounds prevented a variable temperature proton NMR study of their conformation. The calculated total energies reveal that the most stable conformation accessible for DPTCH is cis-cis (I) concordant with the X-ray results in the solid state [6]. The next stable conformation trans-cis (II) lies some 83.7 kJ mol-’ (20.0 kcal mol-’ ) above the lowest energy conformation. The calculations indicate that the other conformation, trans-trans (III) is presumably too sterically hindered to be present in detectable amounts. The calculations for TCH are of interest in that they reveal that a small activation energy is required for the conversion of the cistrans to the trans-trans conformer (2.1 kJ mol-’ or 0.9 kcal mol-’ ). The TABLE

4

Correlationdiagram for crystalline DPTCH Molecular

c 2V

group

Site c,

group

Factor c

zh

group

57

implication is that in the trans-tram conformation of TCH there is less steric repulsion (due to lone pairs on hydrazinic nitrogens) since the -NH2 groups are opposite to each other. The cis-cis conformation is less stable (by 5.4 kJ mol-’ (1.3 kcal mol-’ )) possibly due to dipolar interactions. Electronic

structure

In Table 5, the (Tand n electron densities and the net electron populations are shown. Comparisons between related molecules using the same procedure of calcuIation may be more reliable, while the electron population indices quoted for a single molecule may not be a good guide. On comparing DPTCH with TCH, we find nearly 1.5 electrons have been transferred from the TCH molecule to the protons on protonation with consequent reduction in the formal charges on the protons and nitrogens of -NCH3 groups. It is interesting to note the rearrangement of 0 and n electron populations at all the atoms on protonation. However, the magnitude is larger for the terminal nitrogens, hydrogens and for the sulfur atom and minimal for the atoms of the thioureide skeleton excepting thiocarbonyl sulfur. Thus a significant part of the charge migrated is predicted to come from the terminal NH2 groups and the thiocarbonyl sulfur. Substantial u-7~ rearrangements are also evident for the terminal nitrogens and sulfur. Bond

orders

A comparison of the 7~bond orders of DPTCH with its neutral molecule included in Table 5 demonstrates that in DPTCH, there is a significant TABLE

5

Electron populations

and n bond orders for DPTCH

and TCHa

Atom Electron population DPTCH

n bond order TCH

0

7l

Total

u

R

Total

C N’

4.7039 2.9651 3.2381

1.4621 0.8055 1.8277

6.1660 3.7706 5.0658

H

0.7862

-

0.7862

1.7955 0.6947 1.7211 1.4787 -

N

3.6777

1.2592

4.9369

Hi H,

0.7404 0.7512

-

0.7404 0.7512

4.6675 3.0382 3.3428 3.3394 0.8987 0.8642 4.0317 4.0584 0.9148 0.9135

1.0696 1.0730 -

6.4630 3.7329 5.0639(c) 5.0881(t) 0.8987(c) 0.8642(t) 5.1013(c) 5.1314(C) -

-

0.9148 0.9135

s

=Abbreviations

as in Fig. 1 and Table 1.

Bond

DPTCH

TCH

CS CN

0.731 0.427

0.504 0.571(c) 0.538(t)

NN

0.098

0.135(c) 0.135(t)

58

increase in the C=S bond order with consequent decrease in the C-N bond orders. This result shows up quite clearly in the C=S and C-N bond distances reported for DPTCH [6] : C-S 0.1645 nm and C-N 0.1363 nm as against bond lengths of 0.1724 and 0.1315, 0.1335 run respectively for the neutral molecule [4]. The differences in the C-N bond distances between DPTCH and TCH are relatively less due to the competition between two C-N bonds. These conclusions are also consistent with the findings from solution studies [ 1, 21. The frequencies corresponding to C=S, C-N and N-N stretching modes of TCH and DPTCH cannot be compared since the nature of coupling with other vibrations is different for the two compounds. From the very low NN x bond orders, it is evident that the terminal nitrogens hardly contribute to the conjugation of the TCH molecule. Using a Pauling bond order-bond length relationship [29], the estimate of the percentage of contribution of the resonance structure (I) to the description of bonding in DPTCH is 43% as against 22% in TCH. S II _

I/‘\, _

Torsional

S-

S-

I

I

_+,//“\ N-

-

barrier

The objective of barrier height measurements is the understanding of forces which give rise to restricted rotation [30]. The -N+H3 torsion was found at 526 cm-’ in the spectrum of DPTCH and the method based on the Mathieu equation as illustrated by Fateley and Miller [31] was used for the barrier height calculation. The eigenvalues from Hershbach’s tables 130, 321 were employed for these computations. The F value obtained from molecular parameters is 19.14 cm-i, which yields a barrier of 22.8 kJ mol-’ (5.46 kcal mol-’ ). The torsional barrier calculated using the force constant employing the method detailed by Saito et al. [lS],is 20.5 kJ mol-’ (4.9 kcal n-ml-’ ) and the agreement between the two methods seems very good. Barriers to methyl torsion in methylhydrazine and symmetric dimethyl hydrazine have been studied [30] and the values 13.8 (3.3) to 15.5 kJ mol-’ (3.7 kcal mol-‘) are, as expected, lower than -N+H, barrier heights. The barrier to internal rotation about the C-N bond was calculated [ 181 using the torsional force constant and the energy difference between the cis and trans forms of DPTCH obtained by the CNDO/B method. The value computed is 92.1 kJ mol-’ (22.0 kcal mol-’ ) which compares well with those obtained for thioureas and thioamides .by the NMR method [ 33, 343.

59

REFERENCES 1 G. Braibanti, F. Dallavalle and E. Leporati, Inorg. Chim. Acta, 3 (1969) 459. 2 G. Braibanti, E. Leporati, F. Dallavalle and M. A. Pellinghelli, Inorg. Chim. Acta, 2 (1968) 449. 3 F. Bigoli, A. Braibanti, A. M. M. Lanfredi and A. Tiiipicchio, J. Chem. SOC. Perkin Trans., 2 (1972) 2121. 4 A Braibanti, A. Tiripicchio and M. T. Camellini, Acta Crystallogr., Sect. B, 25 (1969) 2286. 5 A. Braibanti, A. Tiipicchio and M. T. Cameilini, J. Chem. Sot. Perkin Trans., 2 (1972) 2116. 6 A. Braibanti, M. A. Pillinghelli, A. Tiripicchio and M. T. Camellini, Inorg. Chim. Acta, 5 (1971) 523. 7 M. Mashima, Bull. Chem. Sot. Jpn., 37 (1964) 974. 8 K. Dwarakanath, D. N. Sathyanarayana and K. Volka, Bull. Sot. Chim. Belg., 87 (1978) 667. 9 H. W. Gausman, C. L. Phykard, H. R. Hinderlite, E. S. Scott and L. F. Andritch, Bot. Gaz., 144 (1953) 292. 10 C. I. D. Zarafonetis and J. P. Kalas, Proc. Sot. Exptl. Biol. Med., 105 (1969) 560. 11 N. P. Buu-Hoi, T. B. Loe and N. D. Xyong, Bull. Sot. Chim. Fr., (1955) 694. 12 E. Hoggarth, A. R. Martin, N. E. Storey and E. H. P. Young, Br. J. Pharmacol., 4 (1949) 248. 13 G. R. Burns, Inorg. Chem., 7 (1968) 277. 14 F. Bigoli, A. Braibanti, A. M. M. Lanfredi, A. Tiripicchio and M. T. Camellini, Inorg. Chim. Acta, 5 (1971) 392. 15 K. Dwarakanath, D. N. Sathyanarayana and K. Volka, Bull. Sot. Chim. Belg., 87 (1978) 677. 16 M. B. Robin, in S. Patai (Ed.), The Chemistry of the Hydrazo, Azo and Azoxy Groups,

Part 1, Wiley, New York, 1975.

17 G. Agostini, F. Colletta and A. Gambaro, Spectrochim. Acta, Part A, 35 (1979) 733. 18 Y. Saito, K. Machida and T. Uno, Spectrochim. Acta, Part A, 31 (1975) 1237. 19 K. Machida, A. Kagayama, Y. Saito, Y. Kuroda and T. Uno, Spectrochim. Acta, Part A, 33 (1977) 569. 20 J. H. Schachtschneider, Technical Report, Shell Development Company, Emeryville, CA, 1964. 21 J. A. Pople and G. A. Segal, J. Chem. Phys., 44 (1966) 3289. 22 J. A. Pople and D. L. Beveridge, QCPE 141, I n d iana University, Bloomington, Indiana. 23 V. Schettino and R. E. Salomon, Spectrochim. Acta, Part A, 30 (1974) 1445. 24 D. N. Sathyanarayana, K. Volka and K. Geetharani, Spectrochim. Acta, Part A, 33 (1977) 517. 25 K. Geetharani and D. N. Sathyanarayana, Aust. J. Chem., 30 (1977) 1617. 26 R. K. Ritchie, H. Spedding and D. Steele, Spectrochim. Acta, Part A, 27 (1971) 1597. 27 D. Hadzi, J. Kidric, Z. V. Knezevic and B. Barlic, Spectrochim. Acta, Part A, 32 (1976) 693. 28 K. Machida, Y. Kuroda and T. Uno, Spectrochim. Acta, Part A, 30 (1974) 125. 29 S. Merlino, Acta Crystallogr., Sect. B, 25 (1969) 2270.

30 J. R. Durig, S. M. Craven and W. C. Harris, in J. R. Durig (Ed.), 31 32 33 34

Vibrational Spectra

and Structure, Vol. 1, Marcel Dekker, New York, 1972. W. G. Fateley and F. A. Miller, Spectrochim. Acta, 19 (1963) 611. D. R. Herschbach, J. Chem. Phys., 27 (1957) 975. W. E. Stewart and T. I-I. Siddall, Chem. Rev., 70 (1970) 517. C. E. Holloway and M. H. Gitlitz, Can. J. Chem., 45 (1967) 2659.