Vibrational assignments, normal coordinate analysis, B3LYP calculations and conformational analysis...

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Spectrochimica Acta Part A 79 (2011) 1722– 1730

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

jou rn al hom epa ge: www.elsev ier .com/ locate /saa

Vibrational assignments, normal coordinate analysis, B3LYP calculations andconformational analysis ofmethyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate

Tarek A. Mohameda,∗, Ali M. Hassana, Usama A. Solimana,1, Wajdi M. Zoghaibb, John Husbandb,Saber M. Hassana

a Department of Chemistry, Al-Azhar University (Men’s Campus), Nasr City 11884, Cairo, Egyptb Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al Khod, Muscat, Oman

a r t i c l e i n f o

Article history:Received 1 March 2011Received in revised form 5 May 2011Accepted 17 May 2011

Keywords:RamanInfraredNMR spectraVibrational assignmentsDFT calculations

a b s t r a c t

The Raman and infrared spectra of solid methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (MAMPC, C7H8N4S3) were measured in the spectral range of 3700–100 cm−1 and4000–200 cm−1 with a resolution of 4 and 0.5 cm−1, respectively. Room temperature 13C NMR and 1HNMR spectra from room temperature down to −60 ◦C were also recorded. As a result of internal rotationaround C–N and/or C–S bonds, eighteen rotational isomers are suggested for the MAMPC molecule (Cssymmetry). DFT/B3LYP and MP2 calculations were carried out up to 6-311++G(d,p) basis sets to includepolarization and diffusion functions. The results favor conformer 1 in the solid (experimentally) andgaseous (theoretically) phases. For conformer 1, the two –CH3 groups are directed towards the nitro-gen atoms (pyrazole ring) and C S, while the –NH2 group retains sp2 hybridization and C–C N bond isquasi linear. To support NMR spectral assignments, chemical shifts (ı) were predicted at the B3LYP/6-311+G(2d,p) level using the method of Gauge-Invariant Atomic Orbital (GIAO) method. Moreover, thesolvent effect was included via the Polarizable Continuum Model (PCM). Additionally, both infrared andRaman spectra were predicted using B3LYP/6-31G(d) calculations. The recorded vibrational, 1H and 13CNMR spectral data favors conformer 1 in both the solid phase and in solution. Aided by normal coordi-nate analysis and potential energy distributions, confident vibrational assignments for observed bandshave been proposed. Moreover, the CH3 barriers to internal rotations were investigated. The results arediscussed herein are compared with similar molecules whenever appropriate.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Pyrazole is an important heterocyclic molecule with stronghydrogen bonding capabilities. Pyrazole and its derivatives havereceived a great deal of attention since they are used medicallyas antipyretics, anti-rheumatoid agents, as well as herbicides andfungicides in addition to being metal ion extractants [1,2] and cor-rosion inhibitors [3]. They are used extensively for the preparationof biologically active molecules [4,5] with tremendous applicationsin pharmaceuticals as analgesics, anti-inflammatory, anti-bacterial

∗ Corresponding author at: University of Nizwa, College of Arts and Sciences, PostCode 616, P.O. Box 33, Nizwa, Oman. Tel.: +968 202 38503918;fax: +968 202 2629356.

E-mail address: tarek [email protected] (T.A. Mohamed).1 Taken <fn0005>in part from the Ph.D. Thesis of Usama A. Soliman which will be

submitted to Chemistry Department, Faculty of Science, AlAzhar University,Nasr City, Cairo 11884, Egypt.

and anti-depressive agents [6–8]. Recently amino-pyrazole deriva-tives were found to be potentially useful in preventing proteinaggregation which in the human brain is the first phase ofAlzheimer’s disease development [9]. The vibrational spectra ofpyrazole have been thoroughly investigated [9–12], however thereis relatively little information regarding substituted pyrazoles,and N-substituted pyrazoles in particular [13,14]. MP2 [15,16]DFT/B3LYP [17–19] and calculations have become popular towardsstructural elucidation of small and large size molecules [20–22].Furthermore, GIAO NMR DFT-B3LYP calculations aid in the assign-ment of 1H and 13C chemical shifts [23–28].

To the best of our knowledge, the molecular geometry, con-formational stability and vibrational spectra of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (MAMPC,C7H8N4S3) have not yet been investigated either theoretically orexperimentally. To explore the structural parameters and con-formational stabilities of MAMPC we have carried out Gaussian98 [29] and GAMESS [30] calculations utilizing DFT [17–19] MP2[15,16] methods up to 6-31++G(d,p) basis sets. Infrared, Raman,

1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2011.05.044


T.A. Mohamed et al. / Spectrochimica Acta Part A 79 (2011) 1722– 1730 1723

Fig. 1. Infrared (A; 200–4000 cm−1) and Raman (B; 100–4000 cm−1) spectra ofMAMPC.

1H and 13C NMR spectral analysis were performed and aug-mented by theoretical predictions. Complimentary to our priorstudies on CH3 barriers to internal rotations [21,22,31], poten-tial surface scans (PSS) were undertaken for MAMPC. The resultsare reported herein and compared with a recently publishedmanuscript on 5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbothioamide (AMPC, C6H7N5S2) [22].

2. Experimental

All chemicals were purchased from Aldrich Chemical Companywith a purity grade of at least 98%, whereas those used in IR andNMR measurements were of spectroscopic grade.

2.1. Synthesis of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioat

The solid MAMPC sample was prepared by the reaction ofa ketene with a hydrazine derivative. A mixture of 0.01 mol2-bis(methylthio)methylene malononitrile, (SCH3)2C C(CN)2 and0.012 mol of either methyl hydrazinecarbodithioate or benzylhydrazinecarbodithioate in 30 mL absolute ethanol was stirred atroom temperature in the presence of a few drops of triethylamineuntil the thiol evolution ceased. A white brown powder was col-lected and re-crystallized several times from ethanol [32]. The

Fig. 3. 13C NMR spectrum of MAMPC; (A) GIAO-DFT B3LYP/6-311+G(2d,p) calcu-lated spectrum of isomer 1; (B) experimental; (C) inset is the correlation betweencalculated and experimental chemical shifts (ı in ppm).

purity of MAMPC (m.p. 215 ± 1 ◦C) was confirmed by thin layerchromatography (TLC) and mass spectral measurements.

2.2. Infrared, Raman and NMR spectral measurements

The FTIR spectrum of solid MAMPC (4000–200 cm−1) wasrecorded using the CsI disk technique on a Spectrum 100 PerkinElmer spectrophotometer equipped with Spectrum RX software.To obtain a satisfactory signal to noise (S/N) ratio, forty scanswere collected utilizing 1.0 cm−1 resolution, baseline correctionand automatic smoothing features (Fig. 1A). The Raman spectrum(3700–100 cm−1) of solid MAMPC was recorded using a Nexus-670Nicolet Fourier Transform Raman (FT-R) accessories at the NationalResearch Center, Cairo, Egypt (Fig. 1B). The Raman spectropho-tometer is equipped with a 1064 nm Nd:YAG (neodymium-dopedytrium aluminium granet Nd:Y3Al5O12) laser of ∼0.48–0.5 W forexcitation. The observed Raman and IR bands are listed togetherin Table 1. 1H NMR spectrum in DMF-d7 (Fig. 2) and 13C NMRin DMSO-d6 (Fig. 3) were recorded on a Bruker Avance 400 MHzspectrometer equipped with a Magnex Scientific superconduct-ing magnet, variable temperature accessories and Top Spin 1.3software. The freezing point of DMF-d7 is −61 ◦C, therefore lowtemperature 1H NMR acquisitions were carried out successfully atroom temperature down to −60 ◦C (Fig. 4). The LC mass spectrawere acquired on a Quattro Ultima Pt Tandem Quadrupole mass

Fig. 2. 1H NMR spectrum of MAMPC; (A) experimental; (B) GIAO-DFT B3LYP/6-311+G(2d,p) calculated spectrum of isomer 1; (C) inset is the correlation between calculatedand experimental chemical shifts (ı in ppm).


1724 T.A. Mohamed et al. / Spectrochimica Acta Part A 79 (2011) 1722– 1730

Tab

le

1O

bser

ved

and

calc

ula

ted

B3L

YP/

6-31

G(d

) wav

e

nu

mbe

rs

and

pot

enti

al

ener

gy

dis

trib

uti

ons

(PED

s)

for m

eth

yl

5-am

ino-

4-cy

ano-

3-(m

eth

ylth

io)-

1H-p

yraz

ole-

1-ca

rbod

ith

ioat

e

C7H

8N

4S 3

(−d

0),

C6H

2N

4S 3

D6

(2C

D3) a

nd

C6H

6N

4S 3

D2

(−N

D2)

mol

ecu

les.

�ia

Fun

dam

enta

l

C7H

8N

4S 3

mol

ecu

le

C7H

2N

4S 3

D6

mol

ecu

le

(2C

D3)

C7H

6N

4S 3

D2

mol

ecu

le

(ND

2)

Un

scal

edb

Fixe

d

scal

edc

IR

int.

dR

aman

act.

eIR

soli

d(K

Br)

IR

soli

d(C

sI)

Ram

anso

lid

PED

fU

nsc

aled

bFi

xed

scal

edc

PED

fU

nsc

aled

bFi

xed

scal

edc

PED

f

�1

�as

NH

2/N

D2

3674

3506

124.

16

49.5

2

3321

vs

3323

vs

3350

w

88S 1

12S 2

3674

3506

88S 1

12S 2

2719

2595

94S 1

�2

�s

NH

2/N

D2

3502

3342

128.

6415

3.4

3209

m

3211

s

3263

w

88S 2

12S 1

3502

3342

88S 2

12S 1

2535

2421

93S 2

�3

�as

CH

3/C

D3

3178

3133

4.24

95.9

7

(315

9m)

(316

0m)

(316

5

w)

50S 3

50S 4

2358

2324

50S 3

50S 4

3178

3133

50S 3

50S 4

�4

�as

CH

3/C

D3

3165

3120

4.01

77.7

1

(315

9m)

(316

0m)

(316

5

w)

50S 4

50S 3

2347

2314

50S 4

50S 3

3165

3120

50S 4

49S 3

�5

�s

CH

3/C

D3

3084

3040

10.2

5131

.26

(292

0m)

(292

2m)

(293

5

s)

50S 5

50S 6

2331

2273

50S 5

50S 6

3074

3040

50S 5

49S 6

�6

�s

CH

3/C

D3

3076

3032

8.34

186.

28

(292

0m)

(292

2m)

(293

5

s)

50S 6

50S 5

2208

2175

50S 6

50S 5

3076

3033

50S 6

50S 5

�7

�s

C

N

2331

2273

96.7

4402

.14

2228

vs21

70sh

2228

vs21

71sh

2217

vs

88S 7

12S 1

822

03

2172

88S 7

12S 1

823

30

2273

88S 7

12S 1

8

�8

�s

C1

C2

1686

1668

379.

28

6.78

1641

vs

1642

vs

1628

w

21S 8

28S 9

19S 2

4

21S 1

4

1685

1668

20S 8

28S 9

22S 1

4

17S 2

4

1656

1336

35S 8

26S 1

420

S 24

�9

ı Sci

ssor

sN

H2/N

D2

1596

1582

285.

36

41.4

6

(156

0vs)

(156

0vs)

1564

m

47S 9

22S 8

10S 1

015

96

1582

47S 9

22S 8

10S 1

012

24

1212

36S 9

11S 2

110

S 10

10S 1

310

S 20

10S 2

4

�10

�s

C

N

1533

1525

97.2

2

34.7

4

(156

0vs)

(156

0vs)

1530

w

38S 1

021

S 910

S 14

10S 3

0

1532

1523

41S 1

021

S 910

S 24

10S 3

0

1547

1539

46S 1

010

S 13

10S 1

7

10S 3

0

�11

ı ip

CH

3/C

D3

1504

1487

26.3

8

6.31

(148

5s)

(148

5vs)

(150

0w,b

r)

50S 1

139

S 12

1089

1077

48S 1

144

S 12

1504

1487

47S 1

143

S 12

�12

ı ip

CH

3/C

D3

1496

1480

21.9

1

4.99

(148

5s)

(148

5vs)

(150

0w,b

r)

51S 1

240

S 11

1082

1070

46S 1

241

S 11

1496

1480

48S 1

243

S 11

�13

�as

C–N

1453

1441

31.3

5131

.75

1409

m

1410

s

1409

vs

43S 1

310

S 10

10S 1

7

10S 1

810

S 19

1452

1441

44S 1

310

S 17

10S 1

8

10S 1

9

1448

1435

37S 1

314

S 18

12S 1

7

10S 8

10S 1

0

�14

�as

C–N

1398

1379

417.

34

53.0

5

1364

vs

1364

vs

1358

s

11S 1

416

S 24

13S 2

5

12S 1

312

S 16

10S 1

5

10S 1

7

1396

1375

15S 1

420

S 24

15S 2

5

11S 1

311

S 17

10S 1

0

1394

1377

15S 1

428

S 16

14S 1

3

12S 2

510

S 15

�15

ı Um

brel

laC

H3/C

D3

1390

1376

1.63

8.69

1323

w

1323

s13

18w

1329

w

63S 1

537

S 16

1066

1058

46S 1

521

S 20

11S 2

1

10S 1

6

1390

1367

69S 1

531

S 16

�16

ı Um

brel

laC

H3/C

D3

1384

1368

156.

47

54.5

3

1304

s

1305

vs

1314

m

51S 1

630

S 15

1055

1043

75S 1

614

S 15

1382

1365

41S 1

623

S 15

10S 1

4

10S 2

4

�17

�as

C–C

1336

1310

64.6

5

22.5

3

1220

w

1222

s

1279

w

38S 1

724

S 14

15S 8

1336

1316

39S 1

724

S 14

15S 8

1339

1312

39S 1

722

S 14

18S 8

�18

�s

C–C

1222

1211

61.3

9

4.57

1180

w

1207

m

1238

w

39S 1

812

S 25

10S 2

4

10S 3

0

1222

1211

39S 1

812

S 25

10S 2

4

10S 3

0

1163

1151

20S 1

816

S 21

14S 2

5

12S 3

010

S 19

�19

�

NH

2/N

D2

1169

1156

73.1

6

11.7

6

1153

s

1155

s

1172

w

34S 1

920

S 20

17S 3

4

15S 2

112

S 31

1172

1158

34S 1

916

S 20

16S 2

1

16S 3

412

S 31

843

830

38S 1

915

S 26

10S 1

0

10S 1

310

S 29

�20

�s

N–N

1052

1047

43.3

0

21.5

8

1080

vw

1079

vw

1084

w

39S 2

028

S 21

10S 2

310

40

1030

21S 2

017

S 19

10S 8

10S 2

110

S 25

10S 3

0

1109

1098

19S 2

035

S 911

S 34

10S 2

110

S 19

�21

�s

C

S

1036

1024

31.5

2

6.93

1030

m

1030

s

1035

vw

39S 2

115

S 819

S 19

10S 2

510

S 30

1032

1022

15S 2

128

S 15

10S 8

10S 2

010

S 25

10S 2

7

1051

1045

31S 2

141

S 20

10S 2

3

�22

�

CH

3/C

D3

1010

998

18.3

3

15.2

9

(100

0sh

)

(101

0wsh

)

(100

9w,b

r)

47S 2

229

S 23

782

770

47S 2

2

35S 2

310

13

1001

55S 2

228

S 23

�23

�

CH

3/C

D3

1000

988

25.6

0

16.0

9

(100

0sh

)

(101

0wsh

)

(100

9w,b

r)

46S 2

334

S 22

10S 2

080

0

788

37S 2

334

S 22

1000

989

50S 2

327

S 22

10S 2

0

�24

�s

C–N

916

900

115.

32

0.56

910s

910s

894w

12S 2

413

S 26

12S 2

9

11S 1

910

S 10

10S 1

3

10S 2

510

S 33

918

902

12S 2

416

S 26

11S 1

9

11S 3

310

S 10

10S 1

3

10S 2

510

S 29

967

953

15S 2

413

S 25

12S 1

9

11S 8

10S 2

710

S 28

�25

Rin

g

ben

din

g

760

750

67.5

7

18.0

2

750s

750s

761s

28S 2

512

S 30

10S 1

4

10S 1

710

S 20

758

748

27S 2

512

S 30

10S 1

4

10S 1

710

S 20

743

733

25S 2

520

S 30

10S 1

4

10S 1

710

S 20

�26

�as

C–S

722

701

0.53

8.30

(660

w)

(657

w)

715m

61S 2

627

S 29

690

674

49S 2

610

S 18

10S 2

771

7

697

55S 2

634

S 29

�27

�as

C–S

709

688

0.37

10.6

2

(660

w)

(657

w)

636w

52S 2

741

S 28

670

650

49S 2

730

S 28

10S 2

970

8

687

51S 2

745

S 28

�28

�s

C–S

659

647

7.32

12.6

4

(600

wbr

)

(635

w)

(607

w,b

r)

41S 2

824

S 29

10S 3

2

10S 3

4

648

633

30S 2

834

S 29

10S 1

8

10S 3

9

648

638

45S 2

824

S 18

10S 1

7

10S 3

0

�29

�s

C–S

641

631

0.98

3.34

(600

wbr

)

(635

w)

(607

w,b

r)

25S 2

917

S 38

15S 3

9

10S 2

010

S 34

10S 3

5

630

622

34S 2

915

S 38

12S 3

9

10S 8

10S 1

710

S 20

623

612

35S 2

926

S 39

16S 3

5

13S 1

9

�30

Rin

g

ben

din

g

480

470

1.48

2.53

(448

m)

475w

sh

487w

22S 3

023

S 35

10S 3

7

10S 3

810

S 39

478

468

22S 3

023

S 35

10S 2

8

10S 3

210

S 39

470

461

28S 3

018

S 35

10S 2

7

10S 2

810

S 32

�31

ı

C6

S 12

455

445

2.42

23.5

2

418w

420w

419w

25S 3

120

S 20

15S 2

6

15S 2

910

S 37

447

438

30S 3

118

S 20

16S 2

9

14S 2

610

S 30

454

444

31S 3

118

S 20

15S 2

6

15S 2

910

S 37



�32

ı

C1–N

741

5

405

0.01

1.70

–

390v

wsh

404w

15S 3

218

S 28

16S 3

5

12S 3

010

S 25

10S 2

7

412

402

13S 3

220

S 28

15S 3

5

11S 3

010

S 25

10S 2

7

407

396

12S 3

219

S 35

18S 2

8

11S 3

010

S 25

10S 2

7

�33

ı

N5–C

6–S

1331

931

21.

24

2.12

–32

7s31

7m22

S 33

17S 3

116

S 36

10S 3

210

S 37

303

296

25S 3

320

S 32

16S 3

4

14S 3

713

S 38

317

310

17S 3

316

S 31

14S 3

6

14S 3

910

S 29

10S 3

8

�34

ı

C1–N

5–C

630

930

020

.20

2.44

–29

0w28

5vw

22S 3

430

S 32

17S 3

7

10S 3

8

312

305

23S 3

419

S 32

16S 3

1

10S 2

910

S 33

10S 3

6

292

283

13S 3

427

S 37

19S 3

2

16S 3

6

�35

ı

C–C

N28

628

13.

99

6.87

–(2

54w

,br)

260m

30S 3

534

S 31

14S 3

7

12S 3

6

279

247

30S 3

532

S 33

14S 3

7

10S 3

1

283

278

30S 3

531

S 31

13S 3

2

12S 3

4

�36

ı

C3–S

10–C

1126

025

20.

34

0.85

–(2

54w

,br)

230w

45S 3

631

S 33

242

233

59S 3

614

S 33

10S 3

225

624

942

S 36

26S 3

222

S 33

�37

ı

C6–S

13–C

1619

819

310

.19

3.51

––

(178

w)

35S 3

735

S 31

10S 3

4

10S 3

8

184

179

42S 3

732

S 31

197

192

34S 3

734

S 31

10S 3

3

10S 3

8

�38

ı

C3–S

1010

110

01.

00

0.30

––

–33

S 38

29S 3

417

S 31

10S 3

6

9493

44S 3

821

S 34

13S 3

1

10S 3

6

101

101

35S 3

827

S 34

16S 3

1

�39

ı

C2–C

899

983.

39

6.39

––

–50

S 39

31S 3

514

S 38

9898

51S 3

932

S 35

10S 3

499

9951

S 39

32S 3

512

S 38

�40

�as

CH

3/C

D3

3181

3136

2.63

31.6

2

3103

w31

05m

(301

1

m)

51S 4

049

S 41

2361

2328

50S 4

049

S 41

3181

3136

52S 4

048

S 41

�41

�as

CH

3/C

D3

3180

3135

1.91

48.5

4

3000

w29

90w

(301

1

m)

51S 4

149

S 40

2360

2327

50S 4

149

S 40

3178

3135

51S 4

149

S 40

�42

ı as

CH

3/C

D3

1488

1472

9.80

22.8

9

(143

5wsh

)(1

432w

)14

55m

,sh

48S 4

247

S 43

1074

1064

50S 4

248

S 43

1488

1472

48S 4

247

S 43

�43

ı as

CH

3/C

D3

1486

1470

11.1

3

24.7

1

(143

5wsh

)(1

432w

)14

25m

,sh

48S 4

347

S 42

1076

1062

50S 4

348

S 42

1486

1470

48S 4

347

S 42

�44

�

CH

3/C

D3

998

987

3.78

4.24

(980

w)

978m

(986

vw)

54S 4

442

S 45

753

745

68S 4

430

S 45

998

987

54S 4

442

S 45

�45

�

CH

3/C

D3

993

982

3.36

4.98

(980

w)

957w

(986

vw)

54S 4

543

S 44

752

744

68S 4

531

S 44

993

982

54S 4

543

S 44

�46

�

C2–C

872

571

57.

46

1.68

700w

709m

702v

w

30S 4

630

S 54

12S 4

7

10S 5

1

725

715

30S 4

631

S 54

12S 4

8

10S 4

7

724

714

30S 4

621

S 54

13S 4

8

10S 4

7

�47

�

C6

S 12

639

630

1.86

0.51

617w

,br

580v

w

18S 4

722

S 54

14S 6

0

12S 4

811

S 56

639

630

18S 4

722

S 54

14S 6

0

12S 4

811

S 56

636

627

33S 4

713

S 56

14S 6

0

11S 5

410

S 48

�48

�

C3–S

1056

555

30.

01

1.18

(530

vw)

(533

w)

(531

m)

28S 4

833

S 50

19S 4

9

10S 5

1

565

553

28S 4

833

S 50

19S 4

9

11S 5

1

552

544

47S 4

832

S 49

�49

�

C6–S

1353

252

61.

24

0.63

(530

vw)

(533

w)

(531

m)

34S 4

922

S 51

21S 5

0

10S 4

710

S 48

532

526

34S 4

921

S 50

13S 5

1

10S 4

710

S 48

527

523

34S 4

945

S 50

�50

ı tw

ist

NH

2/N

D2

509

500

3.82

3.90

500v

w

451m

510w

44S 5

021

S 51

10S 4

850

9

500

44S 5

020

S 48

11S 5

135

7

354

46S 5

023

S 60

12S 4

7

�51

�

C–C

N

372

372

0.03

0.62

–

369w

365w

50S 5

122

S 50

372

372

50S 5

122

S 47

391

384

64S 5

112

S 54

�52

�

NH

2/N

D2

232

228

52.1

9

1.88

–22

8w,b

r21

5vw

53S 5

220

S 46

231

228

54S 5

236

S 46

142

139

85S 5

2

�53

�

CH

3/C

D3

197

197

5.28

0.08

–

–

198v

w

70S 5

320

S 55

106

104

27S 5

325

S 57

25S 5

8

15S 5

5

196

196

75S 5

324

S 55

�54

�

C1–N

718

418

016

8.45

0.18

––

(178

w)

27S 5

426

S 46

10S 5

2

10S 5

3

184

180

25S 5

441

S 52

25S 4

621

7

214

30S 5

435

S 46

13S 5

1

�55

�

CH

3/C

D3

159

155

0.49

0.06

–

–

146w

49S 5

519

S 53

10S 5

913

9

139

48S 5

542

S 53

160

156

43S 5

521

S 53

19S 5

9

�56

�

C6–N

513

212

90.

67

0.25

––

120v

w30

S 56

25S 5

813

S 55

10S 5

310

S 59

145

144

30S 5

633

S 53

26S 5

9

13S 6

0

131

128

30S 5

624

S 58

18S 5

9

15S 5

7

�57

�

CH

3S/

CD

3S

105

105

0.06

0.33

–

–

–

90S 5

794

94

24S 5

734

S 55

16S 5

9

10S 5

310

S 58

105

105

39S 5

721

S 59

20S 5

8

�58

�

CH

3S/

CD

3S

6464

0.49

0.03

––

–60

S 58

10S 4

610

S 57

5959

62S 5

825

S 57

64

64

60S 5

828

S 57

�9

ı oop

rin

g

pu

cker

47

47

4.82

0.27

–

–

– 43

S 59

27S 5

020

S 60

46

46

40S 5

930

S 60

47

47

42S 5

927

S 60

10S 5

7

�60

ı oop

rin

g

pu

cker

35

35

3.47

1.02

–

–

– 56

S 60

15S 5

410

S 56

34

34

47S 6

030

S 56

20S 5

035

35

57S 6

025

S 56

aFr

equ

enci

es

from

�1

to

�39

belo

ng

to

A′ ,

wh

erea

s

�40

to

�60

belo

ngs

to

A′′ .

bU

nsc

aled

ab

init

io

wav

enu

mbe

rs

for

AM

PTC

uti

lizi

ng

B3L

YP/

6-31

G(d

)

basi

s

set.

cFi

xed

scal

ed

ab

init

io

wav

enu

mbe

rs

uti

lizi

ng

B3L

YP/

6-31

G(d

)

basi

s

set.

For

imp

lem

ente

d

scal

ing

fact

ors

see

Tabl

e

S-1.

dC

alcu

late

d

Ram

an

acti

viti

es

in´ A

4/a

mu

at

B3L

YP/

6-31

G(d

)

basi

s

set.

eC

alcu

late

d

infr

ared

inte

nsi

ties

in

kcal

/mol

at

B3L

YP/

6-31

G(d

)

basi

s

set.

fC

ontr

ibu

tion

s

less

than

10%

are

omit

ted

.



Fig. 4. Temperature-dependent 1H NMR spectra of MAMPC dissolved in DMF from room temperature (25 ◦C) down to −60 ◦C.

spectrometer using chemical ionization (Waters Corp, Milford MA,USA). The sample was initially dissolved in methanol and serialdilutions were carried out in 50/50 (v/v) acetonitrile/water. Thesamples were infused using a Harvard syringe pump (Harvard, CA,USA) at a flow rate of 10 �L per minute into the mass spectrome-ter.

3. Computational procedure

Similar to the closely related AMPC molecule [22], a sum of eigh-teen conformers (Cs symmetry) have been proposed for MAMPC(Supplement Fig. S-1) arising from internal rotations of CH3, CH3Sand NH2 around C–S and C–N, respectively. The LCAO-MO-SCFGaussian-98 calculations [29] were carried out using Density Func-tional Theory (DFT-B3LYP) [16–19] while the Moller Plesset secondperturbation (MP2) calculations [15] where performed using Gen-eral Atomic and Molecular Electronic Structure System (GAMESS)software [30] with basis sets up to 6-311++G(d,p). The MP2 calcu-lations where performed using the High Performance ComputingFacility at Sultan Qaboos University.

3.1. Minimization and frequency calculations

Energy minimization with respect to nuclear coordinates wasachieved by the simultaneous relaxation of all geometric param-eters using the gradient method of Pulay [33]. The optimizedstructural parameters (SPs) were used to estimate vibrational fre-quencies using DFT-B3LYP methods at 6-31G(d) basis set. Quantummechanical (QM) calculations for 1–18 structures favors Conformer1 to be the lowest energy, with the order of stability 1 > 2 > 3(Supplement Fig. S-1). Quantum mechanical (QM) calculations forisomers 1–18 favors Conformer 1 to be the lowest energy, with theorder of stability 1 > 2 > 3. It is predicted to be more stable than 2and 3 by 486 cm−1 (1.39 kcal/mol) and 642 cm−1 (1.84 kcal/mol),respectively. Frequency calculations yield imaginary frequenciesfor all structures except conformer 1, the only stable form whereasother structures represent transition states. Owing to the predictedhigh energies and imaginary frequency predictions for 2–18 iso-mers further investigation was not undertaken (Supplement TableS-1). The predicted SPs for conformer 1 are listed in Table 2 com-pared with microwave [34] and X-ray crystallographic data ofamino-4-cyano-1-phenyl prazole [35]. While B3LYP and MP2 ener-gies for conformers 1, 2 and 3 are given in Table 3.

3.2. Normal coordinate analysis

Normal coordinate analyses were carried out to provide a com-plete description for the fundamental modes of vibrations. Thus,seventy-four independent internal coordinates (Fig. 5) were usedto form sixty symmetry coordinates (Supplement Table S-2) usingthe traditional method of Wilson et al. [36], description for S25, S30,S59 and S60 were taken from Durig et al. [12]. Aided by B3LYP/6-31G(d) calculations, G and F matrices were produced to determinethe B-matrix elements which were used to convert the ab initioFCs in Cartesian coordinates into the FCs in the chosen inter-nal coordinates. The latter enabled us to reproduce the unscaledvibrational frequencies using a program similar to that writtenby Schachtschneider [37]. The unscaled B3LYP/6-31G(d) diagonalforce constants with internal coordinate definitions are listed inSupplement Table S-3. Rational scaling factors (SFs) were usedto obtain fixed scaled FCs to bring the theoretical frequencies in

C1

C2C3

N4

N5N7

C6

H14

H15

S10

C11

H20

H22

H21

C8

S12

N9

L

Σ

τ3τ5

τ2τ1

τ4R

F

AΔ

T1

S

C2 C1

C3

T2

Φ

S13

C16H17

H19H18

C4

α3

α1

β1

β3α2

ερ2

ρ1

ω

σ2

σ1

δ2

δ1

φ2

φ1

λ1ψ2

ψ1

μ2 μ1

η2 η4 r

y

a1

a2

q2

q3

q1

π4

π2

π1

π5

η5

α 6

α 4

β 4 β 6

α 5η1

q 5

q 6

q 4

λ2

π3

β2

β 5

Fig. 5. Atom numbering and Internal coordinates definitions of MAMPC.



Table 2Theoretical (B3LYP and MP2) and Experimental structural parametersa of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (conformer 1).

Parameters MW Ref. [34] X-ray Ref. [35] B3LYP MP2

6-31G(d) 6-311+G(d) 6-31++G(d,p) 6-31G(d) 6-311+G(d) 6-31++G(d,p)

r(C1 C2)b 1.3724 (0.0006) 1.379 (9) 1.396 1.393 1.396 1.389 1.391 1.390r(C2–C3)b 1.4162 (0.0002) 1.405 (5) 1.435 1.435 1.436 1.425 1.428 1.427r(C3 N4)b 1.3306 (0.0005) 1.305 (7) 1.314 1.310 1.315 1.326 1.326 1.329r(N4–N5)b 1.3488 (0.0006) 1.388(5) 1.400 1.398 1.400 1.388 1.381 1.386r(C1–N5)b 1.3591 (0.0001) 1.348 (5) 1.395 1.394 1.395 1.392 1.391 1.392r(C6–N5) 1.384 1.386 1.386 1.396 1.398 1.398r(C1–N7) 1.358 (4) 1.340 1.339 1.341 1.348 1.348 1.350r(C2–C8) 1.408 (7) 1.410 1.406 1.410 1.414 1.413 1.414r(C8 N9) 1.139 (7) 1.166 1.159 1.167 1.185 1.179 1.186r(S13· · ·H17)c 2.348 2.343 2.345 2.338 2.334 1.330r(N5· · ·H15)c 2.666 2.662 2.655 2.682 2.687 2.671r(N9· · ·H14) 3.275 3.281 3.287 3.272 3.266 3.284r(N4· · ·H21) 2.790 2.796 2.803 2.714 2.710 2.708r(S12· · ·H18)c 2.955 2.956 2.959 2.897 2.892 2.903r(S10· · ·H20)c 2.370 2.364 2.366 2.360 2.357 2.353r(S12· · ·H15)c 2.315 2.314 2.290 2.346 2.346 2.319∠ (C1C2C3)b 104.5 (0.02) 105.2 105.2 105.2 105.8 105.6 105.7∠ (C2C3N4)b 111.94 (0.03) 112.2 (4) 112.3 112.1 112.2 111.9 111.7 111.7∠ (C3N4N5)b 104.07 (0.01) 104.4 (3) 105.4 105.6 105.5 105.0 105.3 105.1∠ (N4N5C1)b 113.07 (0.03) 111.8 (3) 111.0 110.8 110.9 111.7 111.7 111.6∠ (N5C1C2)b 106.42 (0.02) 106.2 (3) 106.1 106.1 106.2 105.6 105.7 105.8∠ (C6N5C1) 129.4 (3) 130.8 130.7 130.6 130.9 130.7 130.8∠ (C6N5N4) 118.7 (3) 118.2 118.4 118.4 117.4 117.6 117.6∠ (N7C1C2) 130.5 (3) 128.6 128.6 128.8 128.8 128.5 128.9∠ (N7C1N5) 123.2 (3) 125.3 125.3 125.1 125.6 125.8 125.3∠ (C8C2C1) 127.3 (4) 124.9 124.9 124.9 124.5 124.6 124.5∠ (C8C2C3) 127.4 (4) 130.0 129.8 129.9 129.7 129.8 129.8∠ (N9C8C2) 179.6 (6) 177.6 177.7 177.6 177.3 177.0 177.3� C6S13C16H19 61.3 61.5 61.4 61.3 61.4 61.5� C3S10C11H22 60.9 61.1 61.0 60.8 60.9 61.0A, MHz 711 714 711 718 718 717B, MHz 349 349 348 352 353 352C, MHz 235 235 234 237 237 237�tot, Debye 4.569 4.940 5.027 4.110 4.174 4.178

aBond distances in Å, bond and dihedral angles in degrees, rotational constants A, B, C in MHz and total dipole moment (�tot) in Debye.bMicrowave and X-ray crystallography SPs for pyrazole [34] it self and 5-amino-4-cyano-1-phenylpyrazole [35].cIntra-molecular hydrogen bond between N and S atoms with H atoms, sum of their van der Wall radii is ∼2.75 A and ∼3.00 A, respectively [46].

agreement to the observed infrared and Raman ones [38,39] with acorrelation of 0.998 between scaled and observed frequencies. Wehave also carried out frequency calculations for −2CD3 (MAMPC-d6) and −ND2 (MAMPC-d2) to isolate the CH3 rock, twisting andwagging modes along with the bending modes of amino group(Table 1).

3.3. CH3 barriers to internal rotation

To compare the barriers to internal rotation of MAMPC andAMPC [22], we implemented Potential Surface Scan (PSS) for thetwo methyl groups attached to C–S10 and C–S13 of conformer1 (global minimum) utilizing the B3LYP/6-31G(d) optimized SPs(Table 2). It should be noted that, the hydrogen atoms of the CH3moiety are no longer equivalent because it retains Cs thereforethe local symmetry converts to C1 upon rotating the CH3 groups.For (CH3)A, as the (� H20C11S10C3) dihedral angle is increased, theenergy increases until reaching a maximum at a dihedral angle∼60–70◦ (structure 2) with a barrier of 630 cm−1 (Supplement Fig.S-2). Thereafter, the energy decreases until an angle of ∼120–130◦

is reached corresponding to structure 1′ (C1) (structure 1′ is sim-ilar to 1 except for the out-of-plane non equivalent hydrogen’sH20 and H21). Complete optimizations and frequency calculationswere performed for structures 1′ and 2, in each case an imaginaryfrequency was obtained revealing them to be transition states. Sim-ilarly, (CH3)B was rotated to obtain a minimum in the vicinity ofconformer 1′′ (1′′ is similar to 1) and 3 which reveal imaginaryfrequencies upon complete optimization. The estimated methylbarriers of 630, 480 and 946, 448 cm−1 (Supplement Fig. S-2)

agree well with 722, 689 and ∼703 cm−1 predicted for AMPC [22],trimethyldisilane [31] and trans-trans-2,4-hexadiene [21], respec-tively.

4. Simulated spectra

The combined correlation between the predicted/experimentalIR and Raman spectra firmly assist and provide strong support tothe vibrational analysis of organic molecules [21,22,31].

4.1. Simulated infrared and Raman spectra

To aid and support the forthcoming vibrational interpretations,we calculated the Raman and IR spectra (Supplement Fig. S-3) usingthe B3LYP method and a 6-31G(d) basis set. For the simulatedRaman spectrum, the Raman scattering cross section is derivedfrom the scattering activities and the predicted frequencies whichare proportional to the Raman intensity for each normal mode[40–43]. The IR spectrum was calculated using the dipole momentderivatives; the entire procedure is described by Mohamed et al.[44]. It is worth to mention that, the agreement between IR andRaman calculated/observed frequencies was satisfactory with acorrelation of 0.996 and 0.997, respectively.

4.2. Simulated 1H and 13C NMR spectra

Recently, Gauge-Invariant Atomic Orbital (GIAO) NMR DFT cal-culations have became popular [23] and can successfully predictthe chemical shift (ı, ppm) for small isolated molecules [24,25].



However, the accuracy of NMR theoretical predictions depends onthe implemented basis set and optimized structural parameters.The 1H (Fig. 2) and 13C (Fig. 3) chemical shifts were calculatedusing a Gauge-Invariant Atomic Orbital (GIAO) approach [25] atthe B3LYP/6-311+G(2d,p) level of the theory and referenced toTMS. Solvent effects was also included via the Polarized Contin-uum Model (PCM), the parameters for DMF was taken from Böeset al. [45]. Good correlations of 0.863 and 0.998 were found for theestimated chemical shifts of 1H and 13C NMR, respectively.

5. Structural parameters and force constants

The molecular geometry and structural parameters of MAMPChave yet to be explored by microwave spectroscopy or diffrac-tion techniques making direct comparison between calculated andexperimental difficult. Instead, in Table 2 comparison is made to SPsof pyrazol derived from microwave spectroscopy [34] and to X-raycrystallographic data of 5-amino-4-cyano-1-phenylpyrazole [35].The tabulated values show good agreement between bond- anddihedral-angles and deviations ranging only 0.01–0.04 A in bondlengths. Looking at the calculated values, the calculated C–N bondlength for C1–N7 is found to be shorter than expected for a singlebond implying partial double bond character and sp2 hybridizationfor the nitrogen atom as previously found for AMPC [22]. Compari-son of N· · ·H and S· · ·H distances with the sum of Van der Waal radii[46] suggests the presence of moderate intra-molecular hydrogenbonding as indicated in Table 2. An excellent correlation of 0.9996were found between available X-ray SPs and those predicted fromMP2 and B3LYP using 6-31G(d) basis set. For comparative purposes,the unscaled force constants in internal coordinates of MAMPCwere compared to AMPC [22], identical FCs were predicted forC2C8N9 bending and C1C2C8N9 torsions. Moreover, the stretchingand bendings FCs deviated by ∼4%, whereas wagging, torsions andring puckering FCs deviated by 5–20% (Supplement Table S-3).

6. Vibrational assignment

The vibrational assignment of AMPC [22] certainly aided inthe interpretation of all fundamentals modes of MAMPC althoughfew of them were extensively mixed. Sixty infrared and Ramanactive fundamentals are expected in the vibrational spectra ofMAMPC. Eight vibrations are expected in the high frequency region2900–3500 cm−1 (�1–6, �40 and �41) beside the �C N stretch around2300 cm−1. On the other hand six fundamentals are expected below150 cm−1 which is beyond our instrumental detection capability.Twenty four other fundamentals comprising heavy atom stretchesand bending modes could easily be assigned.

6.1. NH2 fundamental vibrations

Neither the shape nor the position of the N–H stretching modesappears to be strongly affected by hydrogen bonding interac-tions. A similar result was obtained for AMPC compared to theweak broadened features of the Raman spectrum in the regionof 3100–3500 cm−1 (Fig. 1B). The observed IR analogues arepronounced with relatively sharp bands in agreement with the cal-culated infrared intensities (Fig. 1A, Table 1). The IR bands observedat 3323 and 3211 cm−1 (s) respectively were assigned to two NH2stretching modes �1 and �2 in agreement with AMPC [22]. Theunscaled NH2 scissor mode, �9 observed at 1596 cm−1 agrees wellwith a very strong IR band recorded at 1560 cm−1 and as in earlierstudies [22,47] is coincident with the C–N stretching mode. TheNH2 rock (�19; calculated weak Raman activity of 11.7 5 A4/amu)is mixed with 20% N–N (�20) and 15% C S (�21). Therefore, �19 isassigned to the weak Raman band recorded at 1172 cm−1 which

was predicted at 1169 cm−1. Furthermore, the NH2 twisting (�50)and wagging (�52) modes better match the Raman bands observedat 510 cm−1 and 215 cm−1, respectively. It is worth mentioning thatthe ND2 bending modes for MAMPC-d2 undergo frequency shiftsranging from 90 to 375 cm−1 (Table 1).

6.2. CH3 fundamental vibrations

To trace the CH3 and NH2 bending modes below 1700 cm−1

among relatively complex vibrational spectra, we estimated thevibrational frequencies for the deutrated methyl (2CD3) and amino(ND2) isotopomers which are expected to undergo frequency shiftsto lower frequencies (A′; �11, �12, �15, �16, �19, �22, �23 and A′′; �40,�41, �42, �43, �44, �45, �50, �53, �55, �57, �58, Table 1).

The six methyl C–H stretches (A′ and A′′) are assigned to theobserved Raman bands at 3165 (�3 and �4) and 3011 (�40 and�41) cm−1, respectively while the split strong bands at 2935 cm−1 fitthe A′ stretch species (�5 and �6). The predicted bands at 1504 (�11)and 1496 cm−1 (�12) are separated by only 8 cm−1 and both funda-mentals fit the strong IR band at 1485 cm−1. The Raman shoulderrecorded at 1455 cm−1 matches the coincident �42 and �43 funda-mentals predicted at 1488 and 1486 cm−1, respectively. Moreover,the umbrella modes (�15 and �16) match the observed weak andmedium Raman bands at 1329 and 1314 cm−1, respectively. Twomethyl rocking modes (�CH3; �44 and �45 are predicted at 998 and993 cm−1) were observed at 978 and 957 cm−1 in the IR spectrumrespectively. On the other hand, �22 and �23 (�CH3) were over-lapped and therefore assigned to the observed IR band at 1010 cm−1

in agreement with 1013 cm−1 [22,47]. These fundamentals (�22and �23) undergo red shifts of 200–250 cm−1 for deuterated iso-topomers (Table 1). The CH3 torsion modes (�53 and �55) werepredicted at 197 and 159 cm−1 and assigned to the weak Ramanbands at 198 and 146 cm−1, respectively. The CH3S (�57 and �58)torsion modes were predicted at 105 and 64 cm−1 respectively, butwere not expected to be observed because of the Rayleigh scatteringbackground below 100 cm−1 (Fig. 1B).

6.3. Heavy atom fundamentals

The very strong IR/Raman bands at 2228/2217 cm−1 firmly fitthe C N stretch (�7), while the C C stretch (�8) is observed at1642 (vs, IR) and 1628 (w, Raman) cm−1 in good agreement withcalculated infrared intensity (379.28 km/mol) and Raman activity(6.78 A4/amu), Table 1. The C N stretch (�10) and ıip NH2 (�9)vibrational modes were coincident in the IR but resolved in theRaman. Therefore, the observed IR bands were assigned to bothfundamentals (�10 and �9) whereas the weak and medium Ramanbands recorded at 1530 and 1564 cm−1 were assigned to �10 and�9, respectively.

The C–N stretching modes (�13, �14 and �24) reveal exten-sive mixing with five to six vibrational modes (Table 1). The C–Nstretches, �13 and �14, were assigned to 1410(s) and 1364(s) cm−1 IRbands, respectively. These observed bands are at higher frequenciesthan those expected for a C–N single bond indicating partial dou-ble bond character in agreement with the calculated bond length(Supplement Fig. S-1 and Table 2).

The predicted bands at 1336 and 1222 cm−1 were assigned toC–C stretching modes �17 and �18 which fit IR/Raman bands at1222(s)/1279(w) and 1207(m)/1238(w), respectively and are inagreement with the reported spectral range of N-methylpyrazole[13,22]. The observed IR/R bands at 1079(vw)/1084(w) cm−1 wereassigned to the �NN stretch and correlated to a medium bandrecorded at 1056 cm−1 [12]. The A′ ring bending modes, (�25 and�30) were observed at 750(s)/761(s) cm−1, and 475(w, sh)/487(w)cm−1 in the IR/Raman spectra, respectively. On the other hand, we



Table 3B3LYPa and MP2b energies in Hartrees for methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbothioamide (MAMPC; Cs) molecule.

Level and basis set Conformer-1 Conformer-2 Conformer-3 �E1c (cm−1/kcal/mol) �E2

d (cm−1/kcal/mol)

At 6-31G(d) basis setB3LYP level −1685.1041062 −1685.1018921 −1685.1011786 486/1.39 642/1.84MP2 level −1681.9379649205 −1681.9352824015 −1681.9338503955 589/1.68 903/2.58At 6-31++G(d,p) basis setB3LYP level −1685.1393223 −1685.1370774 −1685.1365316 493/1.41 612/1.75MP2 level −1682.0415712101 −1682.0384416107 −1682.0378840152 687/1.96 809/2.31At 6-311+G(d) basis setB3LYP level −1685.3050543 −1685.3026388 −1685.3019796 530/1.52 675/1.93MP2 level −1682.2527844209 −1682.2491632197 −1682.2485690442 795/2.27 925/2.64

aCalculated using Gaussian 98 program utilizing 6-31G(d) basis set (Ref. [29]).bCalculated using GAMESS program utilizing 6-31G(d) basis set (Ref. [33]).c�E1 represents the energy difference between conformer 1 (minimum energy) and conformer 2.d�E2 represents the energy difference between conformer 1 (minimum energy) and conformer 3.

could not observe the out-of-plane ring bending modes (�59 and�60) which were expected below 100 cm−1.

The �C S (�21) is the observed band at 1030/1035 cm−1 in theIR/Raman spectra of MAMPC whereas the ıipC S (�31) observedat 419(w)/420(w) cm−1 in the Raman/IR spectra is calculated(unscaled) at 455 cm−1. The C–S stretches (�26, �27, �28 and �29)are found to be coincident and consistent with the observed Ramanbands at 715 for (�26, �27) and 607 for (�28, �29) cm−1. The ıip C−S(�33, �36, �37 and �38) could be assigned as either CCS or NCS bend-ing which are observed in the Raman spectrum at (317, 230 and178) cm−1, respectively. �38 predicted at 101 cm−1 could not beobserved because of the laser line background. Both ıC6S13C16(�37) and �C1N7 (out-of-plane wagging; �54) are observed at178 cm−1 while they are predicted at 198 and 184 cm−1, respec-tively (Table 1).

7. NMR spectral interpretations

The calculated 13C NMR spectrum (Fig. 3) gives excellent agree-ment with the experimental result thus strongly supporting theoptimized SPs. The 1H NMR spectrum of MAMPC is as expectedvery similar to that of the recently reported spectrum of AMPC[22]. AMPC has two NH2 groups and for these hydrogens its roomtemperature 1H NMR spectrum shows a singlet at 8.9 ppm (in DMF-d7) integrating to 2Hs and two 1H singlets at 9.4 and 9.8 ppm.Thus at room temperature AMPC has one rotatable, and one non-rotatable, C–N bond. Based on the calculated C–N bond lengths,the calculated C–N stretches, and the calculated NH2 barriers tointernal rotation, the rotatable C–N bond in AMPC was assignedas C1–N7 and the signal at 8.9 ppm assigned to H14 and H15 (samenumbering scheme as the present study). In MAMPC the 2H signalat 9.2 ppm is straightforwardly assigned to H14 and H15 therebyconfirming the previous assignment in AMPC and greatly support-ing the calculations on which the assignment was based. Further,while a single peak is observed for these protons in the experimen-tal spectra, in the calculated spectra of both AMPC and MAMPC,H14 and H15 give rise to well separate peaks at 6.0 and 9.3 ppmrespectively. In the calculated spectra then, H14 and H15 are non-equivalent while for the room temperature experimental spectrum,H14 and H15 are equivalent due to free rotation about the C1–N7bond. Low temperature 1H NMR experiments with AMPC showedrestricted rotation about C1–N7 beginning at −45 ◦C. Accordingly alow temperature 1H NMR study of MAMPC in DMF-d7 was con-ducted. Fig. 4 shows a single peak between 9.3 and 9.5 ppm attemperatures down to −50 ◦C becoming broader as the temper-ature is lowered. At −55 ◦C the signal shows two partially resolvedpeaks which are further distinguished at −60 ◦C. Thus the effect ofrestricted rotation about C1–N7 can be clearly seen beginning at−55 ◦C. Further, taking −45 ± 5 ◦C as the coalescence temperatureand the peaks positions of 3880 ± 10 and 3710 ± 10 Hz at −60 ◦C

as the separated peak positions; then the C1–N7 rotational barriercan be estimated using Equation of: �G‡ = −RTc ln[�h�v/

√2KBTc]

[48,49] to be �G‡ = 10.5 ± 0.3 kcal/mol, where R is the gas constant,Tc is the coalescence temperature, h is Planck’s constant, KB is theBoltzmann constant and �� the difference in peak positions.

8. Conclusion

The vibrational and NMR spectra of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (C7H8N4S3) favoredthe existence of conformer 1 in the solid phase, which is ingood agreement with theoretical predictions using the methodsof B3LYP, MP2 and GIAO. In conformer 1, the methyl groups aredirected towards the nitrogen atoms (pyrazole ring) and the C S,while the amino group retains sp2 hybridization and C–C N bondis quasi linear. Novel and complete interpretations were proposedfor the observed IR and Raman bands along with the 1H and 13Cchemical shits.

Acknowledgement

TAM sincerely thanks Professor J.R. Durig, Chemistry Depart-ment, College of Arts and Sciences, University of Missouri, KansasCity, MO 64110, USA, for giving him the opportunity to use G-and F-matrix programs to calculate FCs in internal coordinates andPEDs.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.saa.2011.05.044.

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