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Chemical Physics Letters 432 (2006) 616–622

Dissection of methyl internal rotational barrier in thioacetone

Amrita Chakraborty a, Rina De b,*, Nikhil Guchhait a,*

a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, Indiab Raja Rammohun Roy Mahavidyalaya, Radhanagar, Nangulpara, Hooghly 712406, India

Received 27 May 2006; in final form 25 October 2006Available online 10 November 2006

Abstract

The origin of methyl internal rotational barrier in thioacetone has been studied by relaxation effect, natural bond orbital analysis andPauli exchange interactions. Fully relaxed rotational model, reflecting the true barrier, shows mainly the lengthening of Ccarb–Cme bond,opening of Cme1CcarbCme2 angle and simultaneous wagging of sulfur atom. Calculation shows that the nuclear electron attraction term(DVne) is barrier forming and the principal barrier forming term originates from both the Ccarb–S(r) orbital and lone pair n-orbital. Theopening of Cme1CcarbCme2 angle and lengthening of Ccarb–Cme and Ccarb–S(r) bonds are found to be associated with steric effect andcharge transfer interaction between bonding and antibonding orbitals, respectively.� 2006 Elsevier B.V. All rights reserved.

1. Introduction

The small group rotational barrier in a molecule isimportant in the context of structural and reaction chemis-try and hence, the studies of internal rotational barrier arepopular from the experimental and theoretical point ofviews [1–3]. It is found that the rotational barrier of a singlebond plays crucial role for the functions of carbohydrate,protein and nucleic acids. With the advent of high resolu-tion FTIR, microwave and jet cooled spectroscopy experi-mental information on the dynamics of the rotating groupsare increasingly available. The ability of ab initio calcula-tion in combination with experimental results helps to pre-dict torsional potential energy surface, its shape and barrierheight. Theoretically barrier energy has been explored inthe light of basis set consideration, incorporation of differ-ent correlation effect and the role of multidimensional rota-tional coordinates. Recent theoretical studies have shownthat the roles of skeletal flexing and lone pair reorganiza-tion have important impact on barrier energy and the

0009-2614/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2006.10.107

* Corresponding authors. Fax: +91 33 2351 9755 (N. Guchhait).E-mail addresses: rina04_de@yahoo.com (R. De), nguchait@yahoo.

com (N. Guchhait).

shape of the potential energy surface [4,5]. The ab initiomolecular orbital calculations are being used to investigatethe torsional potential of the flexible molecules. Partition-ing of electrostatic attraction and kinetic energy terms intosymmetry components allow the separation of r and p con-tributions to the barrier. Very recently, natural bond orbi-tal (NBO) analysis has shown further insight to the barrierenergetics [6,7]. It is found that the combination of relaxa-tion calculation with partitioning of barrier energy, NBOanalysis and Pauling exchange interaction provide surgicalprecision of barrier energy and a better understanding ofbarrier origin.

The origin of methyl rotational barrier for single rotorproblem has been explored thoroughly for several systems,such as, the benchmark molecule ethane, propene, acetal-dehyde, thioaldehyde using relaxation calculation, correla-tion effect and NBO analysis [1,7–9]. On the other hand,molecules like acetone, diethyl ether, thioacetone are inter-esting where two methyl groups is considered as doublerotor problem. The gear and anti-gear motion of such dou-ble rotor problem are fascinating to the researcher and theavailable high resolved spectroscopic results on gear andanti-gear motion have been used in combination with the-oretical calculation to find the barrier energy. For double

A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622 617

rotor problem, spectroscopic and theoretical studies ofmethyl rotational barrier for acetone, dimethyl ether andthioether were carried out extensively [5,8–12]. However,very little is known about the corresponding thiocarbonylcompounds. Because of molecular instability thiocarbonyleasily polymerizes and hence the IR spectra of thioacetonehave not been recorded. Kroto et al. has estimated theeffective methyl internal rotation barrier of 455 cm�1 fromthe splitting pattern of microwave spectra [13,14]. Mouleet al. analyzed the methyl rotational dynamics of thioace-tone from pyrolysis jet spectra [15,16]. They estimated rota-tional energy barrier of 1107.88 cm�1 from the analysis ofexperimental results from jet spectra in combination withab initio calculations [15]. Their ab initio calculationsrevealed that the methyl torsional motion in the uppern-p* state is coupled with the wagging motion of thiocar-bonyl sulfur atom. However, till date a details understand-ing of the origin of rotational barrier of thioacetone hasnot been explored as was done in case of acetone and ether.We are interested to know how the sulfur atom causesstructural change on methyl rotation. We also want toexplore individual barrier energetics and compare theresults with its parent molecule acetone. The aim of thiswork is to obtain a physical understanding of the originof the rotational barrier in thioacetone by linking flexingcalculation with symmetry partitioning, NBO analysisand Pauling exchange interaction.

2. Computational methods

All calculations have been done by GAUSSIAN 03 soft-ware [17]. Geometry optimization has been performed atHartree–Fock (HF) and MP2 levels with 6-31+G(d,p)basis set. The C2V symmetry of the equilibrium EE (bottomof barrier) conformer is maintained in the SS (top-of-bar-rier) conformer (Fig. 1). The difference in energy betweenthe SS and EE form is the estimated methyl rotational bar-rier energy. The electronic energy change (DE) is equal tothe sum of change in kinetic energy (DT) and potentialenergy (DV). Total electronic energy change (DE) can beexpressed by following equation [8]:

DE ¼ DT þ DV ee þ DV ne þ DV ee;

where DV ¼ DV ee þ DV ne þ DV ee:

SC

C2

C1

Hop

Hop

Hop

Hop

Hip

Hip

1.6282

1.086

4

114.40

1.5052SC

C2

C1

Hop

Hop

Hop

Hop

Hip

Hip

1.6293

1.0885

117.63

1.5124

1.090

2

121.18

1.086

4

122.80

a b

Fig. 1. MP2/6-31+G(d,p) optimized geometric parameters for thioace-tone (a) top of barrier (SS) and (b) equilibrium (EE) conformers.

At HF level with 6-31+G(d,p) basis set and tight option ofoptimization the virial theorem is satisfied with a discrep-ancy of about 8% of the barrier, i.e. DT = �DE is88 cm�1. We have used this HF calculation with the abovementioned basis set as the calculated barrier is similar tothat of the previous reported barrier and the virial theoremdiscrepency is minimum [15]. Relaxation calculations, NBOanalysis [18] and Pauling exchange terms involving stericinteractions have been carried out at the same HF level[19,20]. For double rotor system, such as thioacetone, boththe methyl groups of the EE form simultaneously rotate by180� to generate the top of the barrier SS geometry. Suchrotation is termed as rigid rotation and the rotation ofmethyl groups followed by full optimization of the rest ofthe geometrical parameters is termed as fully relaxed rota-tion. For the partially relaxation calculation during thecourse of methyl rotation particular geometrical parametersare relaxed to its fully relaxed SS form and the rest of thecoordinates are fixed to its EE value. Both the fully and par-tial relaxations are the flexing calculations as some or allgeometric parameters are allowed to relax in the SS formafter methyl groups rotation. The flexing dependence ofthe energy deconposition terms have been calculated to elu-cidate how the skeletal and methyl relaxation that accom-pany methyl torsional motion affect the individual barrierenergetics. Total exchange repulsion (DEexchange), delocal-ization (DEdelocalization) and Coulomb repulsion (DEee) en-ergy have been calculated by following Ref. [9].

3. Results and discussion

3.1. Geometry

The structure of the equilibrium bottom-of-barrier (EE)and top-of-barrier (SS) conformers of thioacetone moleculeare shown in the Fig. 1. The optimized geometrical param-eters of the equilibrium (EE), top-of-barrier (SS) and otherconformers with respect to methyl rotation obtained by HFand MP2 levels are summarized in Table 1. It is found thatthe fully relaxed calculation at HF level reproduces well thepreviously reported barrier [15]. At the HF level the princi-pal structural relaxations during the course of methyl rota-tion in thioacetone are found to be the lengthening ofCcarb–Cme bond by 0.0062 A and the expansion ofCme1CcarbCme2 angle by 2.99�. At the MP2 level the samestructural relaxations are the lengthening of Ccarb–Cme

bond by 0.007 A and the expansion of Cme1CcarbCme2 angleby 3.2�. The Ccarb–S and Cme1–Hip bonds are also changedbut not so appreciable. In case of acetone, parallel struc-tural changes induced by methyl rotations are the sameCcarb–Cme bond elongation to 0.0052 A and theCme1CcarbCme2 angle opening to 2.9� [8,9]. In case of thio-aldehyde, methyl rotation causes mainly elongation ofCcarb–Cme bond by 0.007 A and CmeCcarbHald angle expan-sion by 1.5� [7]. It is to note that the methyl rotational bar-rier energy in thioacetone (1105 cm�1) is comparativelyhigher than acetone (860 cm�1) [8,9]. The higher barrier

Table 1Optimized geometrical parameters for equilibrium (EE) and top-of-barrier (SS) conformer of thioacetone at HF/6-31+G(d,p) level

Geometrical parameter Bottom (EE) Top (SS) Difference (top–bottom)

Bond a

Ccarb–S 1.6189 (1.628) 1.6220 (1.629) 0.0031 (0.001)Ccarb–Cme1 1.5055 (1.505) 1.5117 (1.512) 0.0062 (0.007)Cme1–Hip 1.0796 (1.086) 1.0828 (1.088) 0.0032 (0.002)

Bond anglea

Cme2CcarbCme1 114.51 (114.4) 117.51 (117.6) 2.99 (3.2)HipCme1Ccarb 111.66 (111.5) 112.45 (112.0) 0.79 (0.5)

Tosional angle (s)

0� 15� 30� 45� 60�

Dihedral angle (�)Cme2CcarbCme1S 180.0 176.16 176.41 174.64 180.0

a Bond length and bond angles are in A and �, respectively. Torsional angle s refers to the rotation of methyl group about Ccarb–Cme bond axis. Values inthe parenthesis are at MP2/6-31+G(d,p) level (barrier at this level is 1102 cm�1). Torsional angle is the angle of rotation of both the methyl groups bysame magnitude from EE form. This is basically the HipCCS dihedral angle.

618 A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622

in thioacetone may be due to larger structural change (i.e.Cme1CcarbCme2 angle and Ccarb–Cme bond relaxation) andgreater polarizability of the sulfur atom relative to that ofoxygen.

Torsional angle dependence of the thiocarbonyl sulfurwagging angle for the ground state is shown in Fig. 2.Focus has been made on the gearing motion where thetwo methyl groups rotate in opposite directions. With fulloptimization of all the geometrical coordinates at eachstage of rotation, it has been found that the sulfur atomadopts a non-planar structure with simultaneous rotationof two methyl groups. For a methyl rotation of 30� themaximum sulfur wagging angle is found to be 5.36�. Mouleet al. found similar observation in the triplet state of thi-oacetone [15]. They have shown that the n–p* excitationshows a progression of large amplitude sulfur waggingmotion in the triplet state. Spectral analysis predicts thatthe Ccarb–S bond is directed out of the Cme1CcarbCme2 plane

60402000

2

4

6

Su

lfu

r W

agg

ing

An

gle

(d

egre

e)

Torsional Angle (degree)

Fig. 2. Dependence of thiocarbonyl sulfur out of plane wagging angle onthe methyl torsional angle (s) at HF/6-31+G(d,p) level. Torsional angle (s)is defined in Table 1.

by 27.36�. This clearly indicates that methyl rotation is cou-pled to sulfur wagging motion. Very similarly the oxygenatom of the parent molecule acetone shows waggingmotion during methyl rotation [8]. In case of single rotorthioaldehyde the aldehyde hydrogen also shows similartype of wagging motion during methyl rotation [7].

3.2. Barrier energies

The small group rotational barrier is found to be con-trolled by different factors, such as basis set consideration,correlation effect and rotational coordinate relaxationeffect [8]. The calculated rotational barrier in thioacetoneat HF level with rigid rotation (1204 cm�1) is higher thanits fully relaxed values (1105 cm�1). As shown in Table 2,the rotational barrier with respect to only Cme1CcarbCme2

angle relaxation reproduces barrier energy comparablewith the fully relaxed value. This indicates that CCC anglerelaxation mainly control the barrier height in thioacetone.As seen in Table 2, other individual relaxation terms suchas Ccarb–S, C–C and Cme1–Hip bonds do not influence bar-rier height as was seen in case of only Cme1CcarbCme2 anglerelaxation. However, the combined relaxation of Ccarb–Cme

and Ccarb–S bonds and Cme1CcarbCme2 angle reproduce bar-rier energy comparable to its fully relaxed rotation. Thepartitioning of barrier energy into kinetic and potentialenergy terms shows that nuclear–electron attraction energy(DVne) in the fully relaxed rotation appears as barrier form-ing term and the trend is similar to that of acetone [8]. It isfound that the barrier energetics of DVne term for doublerotor problem exhibits a strong effect on partial relaxation.The DVne term appears as anti-barrier on Cme1CcarbCme2

angle relaxation. But it appears as barrier forming in caseCcarb–S and Ccarb–Ccarb bond relaxation. On the otherhand, only methyl relaxation shows barrier energetics sim-ilar to that of rigid rotation and hence, it may not reallyaffect the rotational barrier energy. However, overall relax-ation converts an antibarrier DVne obtained in rigid rota-tion to the barrier forming DVne one. Symmetry

Tab

le2

Par

titi

on

of

bar

rier

ener

gyin

tok

inet

ican

dd

iffer

ent

po

ten

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ener

gy(c

m�

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etry

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sect

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laxe

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Can

gle

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xed

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laxe

dc

Met

hyl

rela

xed

CC

and

CC

Cre

laxe

dc

CC

,C

San

dC

CC

rela

xed

c

Bar

rier

(DE

)11

0512

6412

4211

3912

6412

4411

2811

26K

inet

icen

ergy

(DT

)�

1198

4645

�77

047

4626

4860

61�

643

�26

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ucl

ear–

nu

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740

8123

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109

3751

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1603

159

66�

2467

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4885

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lect

ron

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257

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328

3649

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012

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)11

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2056

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331

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473

A2(p

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9027

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9196

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7631

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109

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780

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884

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818

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2912

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ence

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ergy

term

sb

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full

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laxe

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E)

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form

atio

n.

Val

ues

are

nea

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ole

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ith

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nd

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rmer

.

A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622 619

dissection of the electron-nuclear energy changes (DVne)accompanying internal rotation provides deeper insightinto p and r effect on the rotational barrier energetics.The C2V symmetry of equilibrium EE form is retained ongoing to the SS conformation allowing DVne to be brokendown into contributions from A1(r), A2(p), B1(p) andB2(r) orbitals (Table 2). In this type of symmetry categori-zation the A1(r) and B2(r) represent r-lone pair reorgani-zation energy and non-bonding orbital effect, respectively.The A2(p) and B1(p) terms provide information about panti-bonding and bonding interactions. Symmetry parti-tioning of the DVne term shows that A1(r) and B2(r) areonly barrier forming terms leaving all the p terms ofA2(p), B1(p) as antibarrier. The same trend is observedfor rigid, partially relaxed and fully relaxed rotations in thi-oacetone. In case of acetone [8], in addition to the majorbarrier contribution from A1(r) and B2(r) terms theA2(p) term is found to be barrier forming. The largest r-contribution in thioacetone is the A1(r) term whichinvolves lone pair reorganization. The next higher term isthe B2(r) term which accommodates nonbonding orbitaleffect. Interestingly, the effect of individual Ccarb–Cme andCcarb–S(r) bond relaxation on the symmetry dissection ofDVne is found to be same as obtained from fully relaxedrotation. The effect of only Cme1CcarbCme2 angle openingon DVne energetic appears as an antibarrier (Table 2) butits effect on the DVne(r) term is barrier forming (Table 2)and is overwhelmed by the antibarrier DVne(p) compo-nents. In terms of DVne energetics both the A1(r) and B2

(r ) terms are found to be relaxation sensitive and are sim-ilar to that of acetone. The p terms of Vne are found to berelaxation insensitive and appear as antibarrier. The inclu-sion of Ccarb–Cme and Ccarb–S bond relaxation to Cme1C-

carbCme2 angle relaxation on the symmetry dissection ofbarrier energy goes parallel to that of the fully relaxed rota-tion. This indicates that the above mentioned three relax-ations control the rotational barrier in thioacetone.

3.3. Natural bond orbital analysis

A better understanding about the role of different relax-ation on barrier energetics can be obtained by combiningthe symmetry class data with NBO analysis. NBO analysishas an appealing aspect of highlighting the individualbonds and lone-pairs energy that play a vital role in thechemical processes [5–7,21,22]. This scheme enables to dis-sect the barrier energy into bond energy, lone-pair energy,bond–antibond and lone-pair–antibond interactions andsteric repulsions. Table 3 shows the change of bond energy(Dx) accompanying methyl rotation for thioacetone. Thechange in bond energy (Dx) has been obtained from thefollowing relation,

Dx ¼ esqs � eEqE

where es and eE are the NBO energies for the staggered (SS)and eclipsed (EE) conformers, respectively and qs and qE

are the corresponding NBO electron occupancies. From

Tab

le3

Th

eb

on

dan

dlo

ne

pai

ren

ergy

term

inth

ioac

eto

ne

for

rigi

db,

par

tial

lyre

laxe

dc

and

full

yre

laxe

dro

tati

on

alco

nfo

rmer

s(c

m�

1)a

Bo

nd

sR

igid

rota

tio

nC

–Cre

laxe

dC

CC

rela

xed

C–S

rela

xed

Met

hyl

rela

xed

CS

and

CC

Cre

laxe

dC

Can

dC

CC

rela

xed

CC

,C

San

dC

CC

rela

xed

Fu

lly

rela

xed

Cca

rb–S

(r)

534

229

2202

1970

530

3624

1867

3287

3319

Cca

rb–C

me1

�18

518

62�

1026

�31

3�

234

�11

5710

3290

684

9C

me1

–Ho

p18

0715

9416

0517

6213

2915

5613

9113

4285

9L

on

ep

air

(1)

1061

872

1077

1072

964

1096

896

915

815

Lo

ne

pai

r(2

)20

9020

5119

2320

4020

6018

7418

8118

3218

03

aDx

(Cal

cula

ted

fro

meq

uat

ion

inth

ete

xt),

the

po

pu

lati

on

wei

ghte

dN

BO

ener

gyd

iffer

ence

for

the

SS

and

EE

con

form

ers,

valu

esar

en

eare

stw

ho

len

um

ber

s.b

,cS

ame

asfo

otn

ote

ban

dc

of

Tab

le2.

620 A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622

NBO analysis it is found that the highest contribution tothe barrier arises from Ccarb–S (r) bond and second highestfrom the lone pair (n) orbital in the fully relaxed rotation.The other barrier forming terms originate from Cme–Ccarb,Cme-Hop bonds. But their contribution to barrier is lesscompared to Ccarb–S(r) bond. On the other hand, the high-est barrier forming term in acetone is C–Hop bond and thesecond one is the lone pair (n) orbitals [8]. The NBO energyassociated with the highest barrier forming Ccarb–S(r) orbi-tal is mainly controlled by C–S bond and CCC angle relax-ation. The sulfur nonbonding orbital and C–Hop bond arefound to be relatively insensitive to skeletal relaxations.Where as the barrier forming term from Ccarb–Cme orbitalssolely depend on C–C bond stretching but not on CCC an-gle opening or C–S bond stretching. However, the relaxa-tion of Ccarb–Cme and Ccarb–S bonds and Cme1CcarbCme2

angle effectively bring the NBO energy changes comparableto the fully relaxed values. Therefore, NBO analysis alsosupports that C–C and C–S bonds and CCC angle relaxa-tion are found to play important role in controlling thebarrier energy in thioacetone.

3.4. Bonding–antibonding interaction

The hyperconjugative interaction between the bondingand antibonding orbital can explain some conformationeffect such as bond elongation [7,23,24] that accompanyinternal rotation. The bond–antibond and lone-pair–anti-bond interactions have been calculated by following theWeinhold‘s method of evaluation of F �2ij /(ei � ej) using sec-ond order perturbation theory [25], where F �ij is the Fockmatrix element between the bonding (or lone pair) NBOand a virtual unoccupied antibonding orbital. The princi-pal bond–antibond and lone-pair–antibond interactionenergies are given in Table 4. The implication of this typeof interaction lies in their associated bond/antibondingcharge transfer leading to modification of bond length.The significant barrier forming bond–antibond interactionarises from Cme1–Hip(r)/Ccarb–Cme2(r)* and Cme1–Hop(r)/Ccarb–S(r)* terms. Other bond–antibond interaction termsare comparatively less important. Interestingly the relaxa-tion has no effect on bond–antibond interaction terms. Incase of acetone [8], the barrier forming bond–antibondinteraction is Ccarb–S(p)/Cme1–Hop(r)* and this interactionenergy is only 300 cm�1. This interaction in acetone is lesscompared to thioacetone (Table 4). The interactionbetween Cme1–Hip(r) bonding and Ccarb–Cme2(r)* anti-bonding orbitals for the ground state EE conformer pro-vides significant overlap of the bonding and antibondingorbitals (Fig. 3a) in the Ccarb–Cme bond region leading toa charge transfer interaction (Fock matrix elementF �ij = 0.070 a.u.). The same orbitals for the SS conformerdoes not show the above type of overlap of bonding andantibonding orbital (Fig. 3b) and hence F �ij (0.0 a.u.)reduces significantly. The next barrier forming term is theinteraction between Cme–Hop(r) bonding and S–Ccarb(r)*

antibonding orbital (Fig. 4). For these interactions the

Table 4Principal barrier forming bond–antibond and Pauli exchange repulsion interaction terms in thioacetoneab

Fully relaxed Rigid rotation C–C relaxed C–S relaxed CCC relaxed Methyl relaxed

Bond–antibond interaction (donor–acceptor)

Cme1–Hop(r)/Ccarb–S(r)c 1014 1000 1000 1000 1014 1000Ccarb–S(p)/Cme1–Hop(r)c 430 427 437 427 420 430Ccarb–Cme2(r)/Cme1–Hip(r)d 686 675 675 675 682 679Cme1–Hip/Ccarb–Cme2(r)d 1567 1560 1563 1563 1563 1560Ccarb–S(n)/Cme1–Hip(r)c 430 426 437 426 420 430

Pauli exchange repulsion

Ccarb–S(r)/Cme1–Hip 1329 1308 1263 1322 1361 1312Cme1–Hip/Cme2–Hip 539 979 941 979 605 916Ccarb–S(p)/Cme1–Hop 143 147 108 143 192 143

a Same as footnote a of Table 2.b Contribution of only one is given.c There are four such interactions.d There are two such interactions.

Fig. 3. Orbital contour diagram for thioacetone Cme1–Hip bonding andCcarb–Cme2* antibonding pre-NBO in (a) EE and (b) SS conformers.

A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622 621

value of F �ij is 0.056 a.u. for the EE form and 0.0 in the SSform. Here also there is a favourable overlap between theCme1–Hop(r) bond and S–Ccarb(r)* antibonding orbital inthe Ccarb–Cme bond region for the EE form compared tothat of the SS form. Similarly there is charge transfer inter-action from Ccarb–Cme2(r) bonding orbital to Cme1–Hip(r*)antibonding orbital and hence it has some contribution tothe barrier energy. All these three interactions showfavourable overlap of bond and antibond orbital at theCcarb–Cme bond region.

Fig. 4. Orbital contour diagram for thioacetone Cme1–Hop bonding andCcarb–S(r)* antibonding pre-NBO in (a) EE and (b) SS conformers.

Weakening of the Ccarb–Cme bond can thus be explainedby considering favourable bonding and antibonding over-lap in the Ccarb–Cme bond region for the EE form com-pared to that of the SS one (see contour diagram inFig. 3,4). Therefore, the lengthening of C–C bond accom-panying internal rotation in thioacetone is due to delocal-ization interaction between bonding and antibondingorbitals. On the other hand, the Ccarb–S(r) bond lengthen-ing may be due to electron transfer from C–S(r) bondingorbital to Cme1–Hop antibonding orbital. All the bond–antibond interactions involving p electrons (data not given)are found to be antibarrier, which rationalizes the antibar-rier A2(p) and B1(p) terms of DVne energy.

3.5. Steric repulsion

As shown in Table 4, the main barrier forming Pauliexchange terms calculated by using Badenhoop and Wein-hold formulation are two Ccarb–S(r)/Cme1–Hip bonds. Verysimilar type of steric repulsion term was reported for ace-tone and thioaldehyde also [7,8]. However, this interactionis quite less in acetone (500 cm�1) compared to that of thi-oacetone (1329 cm�1). The Cme–Hip/Cme–Hip (i.e. methyl–methyl repulsion) Pauli exchange repulsion energy(539 cm�1) is found to be of less contributing term thanCcarb–S(r)/Cme1–Hip repulsion term and this interactionenergy is very much similar to that of acetone (500 cm�1)[9]. Relaxation has no effect on the Ccarb–S(r)/Cme1–Hip

interaction term. On the other hand, the methyl–methylrepulsion (Cme–Hip/Cme–Hip) is sensitive to relaxationand it shows more effect on Cme1CcarbCme2 angle opening.Therefore, the expansion of the Cme1CcarbCme2 angle atthe top of the barrier can be explained by the increase ofmethyl–methyl steric repulsion between Cme–Hip bonds ofthe two methyl groups. The smaller CCC angle in the thi-oacetone (114�) equilibrium conformer than that of ace-tone (116�) equilibrium conformer is giving an indicationof increase Ccarb–S(r)/C–Hip repulsion in thioacetone.Pophristic et al. [9] have shown in details hyperconjugativeinteraction depending on overlap between bonding and

Table 5Energy analysis to fully internal rotational barrier at HF/6-31+G(d,p)level (kcal/mol)

DEexchange DEdelocalization DEee

�2.97 4.92 (3.17) �109.78

DEexchange, DEdelocalization and DEee are the exchange repulsion, delocal-ization and coulomb repulsion energy changes, respectively (Ref. [9]).Value in the parenthesis is for acetone at HF/6-31+G(d,p) level.

622 A. Chakraborty et al. / Chemical Physics Letters 432 (2006) 616–622

antibonding orbital. These hyperconjugative interactionskeep the CCC angle at a lower value than if these areabsent. By the method of deletion of antibonding orbitalthey have shown that oxygen Lp(n)/C–C* interaction isthe principal hyperconjugative interaction in acetone. Inthe similar molecule like thioacetone (oxygen is replacedby sulfur) the Lp(n)/C–C* interaction is observed to be lar-ger (F �ij = 4.92 for thioacetone) compared to acetone(F �ij = 2.19 for acetone). This increased bond/antibondinginteraction may also be another cause of small Cme1

CcarbCme2 angle in thioacetone.In order to make a comparison of rotational barrier

energetics between acetone and thioacetone, an energyanalysis for the fully relaxed internal rotation of thioace-tone is given in Table 5 where the total contribution ofexchange repulsion, delocalization and Coulombic repul-sion to the barrier have been considered. As seen in Table5, the only barrier forming term is the delocalization energyand both the exchange and Coulomb repulsion terms areappeared to be antibarrier. The complete optimizationstudy of acetone has revealed the same trend of stabilizingPauling exchange and Coulomb repulsion and destabilizingdelocalization terms [9]. The value in the parenthesis inTable 5 is the delocalization energy for acetone derivedwith HF/6-32+g(d,p) optimization. Hence it can beinferred that the higher change of delocalization energymay be responsible for the higher barrier energy in thioac-etone than acetone.

4. Conclusions

In this Letter, rotational barrier energetics of thioace-tone have been reported and compared with its parent mol-ecule acetone. The major structural change (elongation ofC–C bond and CCC angle) and barrier energetics in thioac-etone is similar to that of acetone. However, the magnitudeof barrier and different individual barrier contributingterms are found to be higher than that of acetone. Thecombined results of symmetry partitioning, NBO andrelaxation analysis demonstrate the important role of r-lone pair, sulfur nonbonding and C–C(r) bond in control-ling barrier height. For an in-depth understanding thevarious contributions of Pauli‘s exchange repulsion, delo-calization and electrostatic repulsion energy to the barrierhave been considered. In term of pairwise exchange repul-sion, the lone pair/C–Hip bond repulsion energy changeand methyl–methyl repulsion energy change are barrier

forming but the total repulsion change is found to be anti-barrier. Coulomb repulsion energy change is also appearedto be antibarrier. However, delocalization energy is foundto be barrier forming. It is not the steric repulsion butthe delocalization energy is found to be responsible forthe higher barrier energy in thioacetone compared to ace-tone. The small Cme1CcarbCme2 angle in thioacetone com-pared to acetone has been explained by the largestabilizing Lp(n)/C–C* hyperconjugation interaction. Theexpansion of Cme1CcarbCme2 angle is found to be due tomethyl–methyl steric repulsion. The weakening of theCcarb–Cme bond is due to bond/antibonding interactionproviding a favorable bonding overlap in Ccarb–Cme bondregion for the eclipsed conformer than that of the staggeredone.

Acknowledgements

N.G. and R.D. thank DST, India (Project No. SR/S1/PC-1/2003) and UGC, India for financial support, respec-tively. The authors thank the reviewer for several criticalcomment and suggestion.

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