Molecular structure and conformational preferences of methylthiodichlorophosphite, Cl2PSCH3, as studied by gas electron diffraction and quantum-chemical calculations

Journal of Molecular Structure 978 (2010) 4–10

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Molecular structure and conformational preferences of methylthiodichlorophosphite, Cl2PSCH3, as studied by gas electron diffraction and quantum-chemical calculations A.V. Belyakov a,*, A.N. Khramov a, V.A. Naumov b a b

Saint-Petersburg State Technological Institute, 190013 St. Petersburg, Russia Arbuzov Institute of Organic and Physical Chemistry, Kazan, Tatarstan, Russia

a r t i c l e

i n f o

Article history: Received 30 April 2009 Received in revised form 28 May 2009 Accepted 28 May 2009 Available online 23 June 2009 To memory of Professor Victor A. Naumov. We dedicate this paper to Professor Heinz Oberhammer in connection with his 70th birthday Keywords: Phosphites Molecular structure Gas electron diffraction Anomeric effect NBO analysis

a b s t r a c t Free Cl2PSCH3 molecule has been studied by gas electron diffraction (ED), B3PW91/6-311+G* (DFT) and MP2/AUG-cc-PVTZ quantum-chemical calculations. Structure optimizations of Cl2PSMe molecule by both theoretical methods (DFT/MP2) indicate that the most stable conformer is an anti conformer of Cs symmetry, while the energy of a gauche conformer is about 1.3/1.6 kcal mol–1 higher. Each conformer is characterized by dihedral angle s(CSPlp) where lp denotes the direction of the electron lone pair on the P atom; assumed to lay in the plane passing through P–S bond and bisector of the ClPCl bond angle. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are: v(anti) = 65/79%, v(gauche) = 35/21%. Experimental ED data agree well with joint presence of both conformers in the ratio v(anti) = 68(12)% and v(gauche) = 32(12)%. Natural Bond Orbital (NBO) analysis suggests that the relative stabilities of the two conformers as well as the differences between bond distances, valence angles and different NBO descriptors may be determined by anomeric effects. The most important of which is plpS ? rPCl , that is delocalizations of p lone pair of the S atom into antibonding orbital of P–Cl bond. Ó 2009 Published by Elsevier B.V.

1. Introduction Thioesters of acids of three coordinated phosphorous differ strongly by chemical behavior from their oxygen analogous the structures and properties of which are studied quite comprehensively, but the structures of thioesters in the gas phase particularly are studied to a much lesser extent [1]. The aim of the present contribution is to determine molecular structure and conformational stability of gaseous Cl2PSMe molecule by electron diffraction method and quantum-chemical calculations. In order to increase our understanding of the conformations adopted by the free molecule we also include the results of NBO analysis [2,3]. The simplest molecules containing a single bond between P and S atoms, i.e. H2PSH and F2PSH, have been studied by ab initio molecular orbital calculations in Ref. [4]. Both of them possess two stable conformations in which S–H bond is located synperipla-

* Corresponding author. Address: Department of Analytical Chemistry, SaintPetersburg State Technological Institute, Moskovskii prosp. 26, 190013 St. Petersburg, Russia. Tel./fax: +7 812 3162991. E-mail address: [email protected] (A.V. Belyakov). 0022-2860/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.molstruc.2009.05.058

nar and antiperiplanar relative to the lone electron pair of P atom (lpP). It was found that the syn structure is preferred in compound with equivalent substituents, whereas the anti form is preferred for mixed substituents. According to the nomenclature rules of IUPAC the dihedral angle s(HSPlp) describing relative orientation of two ends of the molecules should be defined as 0° in syn and 180° in anti [5,6]. The analogous molecules with methyl groups Me2PSMe and F2PSMe were studied experimentally and theoretically in Refs. [4,7–11]. As before for free molecules the syn or near syn structure is preferred in compound with equivalent substituents, whereas the anti form is preferred for mixed substituents. The authors of ED study of the molecule F2PSMe found that the most favored structure is that where dihedral angle s(CSPlp) is 106(9)° though the structures with the angles 19(3) and 171(5)° fit the observed data almost equally well [11]. Unfortunately the authors of the previous ED study of the molecules of the series MeSPX2, X = Cl and Br were also unable to determine unambiguously conformational composition on the basis of ED data alone [12]. At so doing some restrictions were imposed on bond lengths and bond angles which, as latter turned out, are inconsistent with theoretical findings. In Ref. [13] the molecule Cl2PSMe was studied by ab initio and

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ple-zeta AUG-cc-PVTZ basis set augmented by diffuse functions [18,19] and computer programs [20,21]. 3. Gas electron diffraction

Scheme 1.

variable temperature infrared spectroscopy methods and was found that anti conformer is the more stable form. To resolve contradictions we decided to reinvestigate previous ED data on the Cl2PSMe molecule with the use of additional quantum-chemical data on the force fields and vibrational amplitudes. 2. Quantum-chemical calculations All calculation were carried out at the B3PW91/6-311+G* (DFT) and MP2/AUG-cc-PVTZ (MP2) levels of theory using Gaussian 03 program system [14]. The investigation revealed three minima on potential energy hypersurface: anti, gauche+ and gauche– (see Scheme 1). Structure optimization of the anti conformer of Cl2PSMe molecule was carried out under Cs symmetry and optimization of the gauche conformer without imposition of symmetry. Calculations of the molecular force fields confirmed that the optimal structures thus obtained correspond to minima on the potential energy hypersurface. The gauche conformer corresponds to two degenerate minima gauche+ and gauche– which are indistinguishable by structural analysis. The theoretical molecular force fields were used to calculate mean vibrational amplitudes (u) and vibrational correction terms D = rh1 – ra using a program [15]. NBO [2,3] and AIM [16,17] analysis of SCF wavefunction was carried out with the use of Dunning’s correlation consistent tri-

Methylthiodichlorophosphite was obtained as published in Ref. [22], and had bp 38–40 °C at 10 mm Hg. The purity of the samples was checked by NMR method and was not worse than 95%. The electron diffraction patterns were recorded in Kazan on EG-100A electron diffraction apparatus using cubic sector at nozzle temperature 20 °C and accelerating voltage of 40 kV. The electron wavelength was calibrated against the bond distance in NH4Cl. Four and three photographic plates for long (365 mm) and short (186 mm) nozzle-to-plate distances, respectively, were scanned on the GIII Zeiss microdensitometer and the data were processed as described in Ref. [23]. Atomic scattering factors were taken from Ref. [24]. Experimental backgrounds were drawn as cubic spline functions to the difference between experimental and theoretical molecular intensity curves using a program written by A.V. Belyakov. The experimental intensity data extended from 3.6 to 12.2 Å–1 and from 11.4 to 29.2 Å–1 for the long and short nozzle-to-plate distances, respectively. Both curves are with an increment of 0.2 Å–1. The three P–S, C–S, and C–H distances were refined as independent parameters; differences between chemically equivalent but symmetry inequivalent bond distances (e.g. P–Cl and C–H distances) in the same conformer or between chemically equivalent bond distances in different conformers as well as differences between close parameters (e.g. P–S and P–Cl) were fixed at the values indicated by MP2 calculation. Similarly the bond angles \SPCl and \SCH were refined as independent parameters, while differences between chemically similar but symmetry inequivalent angles were fixed at calculated values. Due to the strong correlations bond angles \ClPCl and \CSP were refined in group of \SPCl bond angle with fixed theoretical differences. The dihedral angle characteriz-

Table 1 First two columns; Nondegenerate minima on the potential energy surface of Cl2PSMe molecule obtained by DFT calculations at the B3PW91/6-311+G* level vs. MP2/AUG-ccPVTZ data. Relative electronic energies at zero K (DE); relative standard enthalpies (DH°), relative standard free energies (DG°) and mole fractions (v) in the gas phase at 298 K; bond distances, bond angles and dihedral angles. Last two columns; relative standard free energies (DG°); mole fractions of the conformers; bond distances, bond angles and dihedral angles of the anti and gauche, conformers obtained by least-squares refinement to the gas electron diffraction data (ED). Conformer (symmetry)

DFT/MP2 anti (Cs)

ED gauche (C1)

anti (Cs)

gauche (C1)

v

0/0 0/0 0/0 0.65/0.79

1.31/1.64 1.29/1.62 0.77/1.21 0.35/0.21b

0 0.68 (12)

0.86 (37) 0.32 (12)

Bond distances (in Å) P–S P–Cl P–Cl0 S–Ce C–Hd

re 2.106/2.087 2.102/2.076 2.102/2.076 1.822/1.815 1.090/1.087

re 2.122/2.099 2.111/2.082 2.079/2.058 1.826/1.822 1.090/1.087

ra e 2.059 (5) 2.047 2.047 1.765 (11) 1.116c

ra 2.070 2.053 2.030 1.773 (11) 1.116c

Bond angles (in °) PSC SPCl SPCl0 ClPCl0 SCH (anti) SCH (gauche)

\e 105.6/103.7 103.4/102.3 103.4/102.3 99.0/98.2 106.0/106.0 110.6/109.8

\e 97.0/95.4 104.1/102.3 97.2/96.3 101.2/100.7 105.9/106.2 111.4/110.4d

\h1 103.7 102.2 102.2 98.2 106.0c 109.8c

\h1 95.3 102.2 96.3 100.7 106.2c 110.4c,d

Dihedral angles (in °) s = CSPlp CSPCl

se

se

sh1

sh1

180.0/180.0 51.4/50.7

48.1/53.8 –82.9/–77.4

180.0 50.7

53.8 –77.4c

–1

DE (kcal mol ) DH°298 (kcal mol–1) DG°298 (kcal mol–1)

a b c d e

a

The electronic energy of anti conformer is –1699.79476821/–1697.9299844 au. Racemic mixture. Fixed in accordance with experiment. R-factor = 4.0% was calculated on Eq. (1). Mean value. Parenthesized values are 3r. Refined in groups with differences fixed on theoretical values (see text).

(3)

(5)

(3)

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ing rotation of PSMe group was fixed at the value obtained from the MP2 calculations. Finally we refined the mole fractions of the anti conformer while the mole fraction of gauche conformer was calculated as the difference v(gauche) = 1 – v(anti). The mean amplitudes were refined in three groups with constant differences: amplitudes for bonded distances, amplitudes for non-bonded distances independent to rotation around P–S bond and amplitudes dependent to rotation around P–S bond. When refining structural parameters the minimized functional has the form:

Q¼

X

ws D2s ¼

X

s

h i2 ws sMobs ðsÞ  k  SMcal ðsÞ

s

where ws is a weight function; s = (4p/k)sin(h/2) is parameter of scattering angle h; k is wavelength of electron beam; sM(s) is molecular intensity curve and k is the scale factor. As a criterion of minimum of the functional serves the value of R-factor:

, R¼

Q

X

 2 ws sMobs ðsÞ

Fig. 1. Experimental (dots) and calculated (solid line) molecular intensity curves of Cl2PSMe molecule. Below: difference curve.

!1=2 ð1Þ

s

Least-squares structure refinements were carried out with a modified version of the program KCED25 [25]. Weight matrices were diagonal, the short distance data were assign half weight and the long distance data weight was as follows:

h i 1 ws ¼ 1:0  exp 0:67  ð5:2  sÞ2 for s < 5:2 Å ws ¼ 1:0 for 5:2 6 s 6 12:2 Å

1

Estimated standard deviations calculated by the program were multiplied by a factor of three to include added uncertainty due to data correlation and an estimated scale uncertainty of 0.1%. The final set of structural and vibrational parameters for anti and gauche conformers is listed in Tables 1 and 2. Observed and calculated molecular intensity and radial distribution curves are compared in Figs. 1 and 2. 4. Results and discussion Theoretical optimization of geometry of Cl2PSMe molecule led to the identification of two distinct conformers, the most stable

Fig. 2. Experimental (dots) and calculated (solid lines) radial distribution curves of Cl2PSMe molecule. Below: difference curve.

of them being anti conformer of Cs symmetry. The calculations revealed two degenerate minima corresponding to two enantiomeric gauche± conformers of C1 symmetry at the higher energy (see Scheme 1) which are undistinguishable in ED structural analysis. Structural parameters corresponding to the refined models are given in Table 1. The calculated standard free energies at 298.15 K

Table 2 Experimental structural parameters (ED) of Cl2PSMe molecule (bond lengths ra in Å, amplitudes u in Å, and vibrational corrections rh1 – ra in 10–4 in Å) and ab initio values for geometric and vibrational parameters. Parameters

ED

MP2/AUG-cc-PVTZ

anti

gauche

ra

u

P–S P–Cl P–Cl0 S–C C–Ha C–H0 C–H0 0

2.059 2.047 2.047 1.765 1.116 1.115 1.115

0.049 0.049 0.049 0.050 0.074 0.074 0.074

ClCl0 SCl0 SCl

3.091 3.193 3.193

0.079 0.076 0.076

CCl CCl0 PC

3.319 3.319 3.007

0.196 0.196 0.074

a b

anti

gauche

ra

u

re

u

rh1 – ra

re

u

2.070 2.053 2.030 1.773 1.116 1.116 1.117

0.049 0.050 0.047 0.050 0.074 0.075 0.075

2.087 2.076 2.076 1.815 1.087 1.086 1.086

0.051 0.051 0.051 0.052 0.076 0.076 0.076

1 3 3 2 16 16 16

2.099 2.082 2.058 1.822 1.087 1.087 1.088

0.050 0.052 0.049 0.052 0.076 0.076 0.076

2 4 2 3 17 16 16

(6)

3.139 3.050 3.205

0.084 0.089 0.084

(6)

3.139 3.242 3.242

0.090 0.087 0.087

33 30 30

3.188 3.097 3.255

0.095 0.099 0.095

37 35 46

(31)

3.538 4.484 2.841

0.280 0.099 0.097

(31)

3.384 3.384 3.074

0.188 0.188 0.085

74 74 48

3.629 4.597 2.906

0.272 0.091 0.107

291 261 73

b

(2)

(2)

C–H for H atom in anti position relative P–S bond, C–H0 and C–H00 are for s(PSCH0 ) > 0° and s(PSCH00 ) < 0°, respectively. Parenthesized values are 3r. Refined in groups with differences fixed on theoretical values (see text).

rh1 – ra

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(DFT/MP2) indicate that the mole fractions in the gas phase at this temperature are: v(anti) = 65/79%, v(gauche) = 35/21%. ED analysis, augmented by theoretical calculations of force constants, vibrational amplitudes, shrinkage corrections and small theoretical differences in similar structural parameters, permitted to determine geometry and conformational composition of Cl2PSMe molecule. Experimental ED data agree well with joint

7

presence of both conformers in the ratio v(anti) = 68(12)% and v(gauche) = 32(12)%. Molecular intensity curves for the best set of geometrical parameters from Table 1 are shown in Fig. 1. Observed and calculated radial distribution curves for the mixture of anti and gauche conformers are presented in Fig. 2. Refinements for anti and gauche conformers alone lead to increase in R-factor of about 0.6% and 6.5%, respectively.

Fig. 3. 3D NBO and 2D NBO contours for hyperconjugative donor–acceptor interactions in gauche (a) and anti (b–d) conformers of Cl2PSMe molecule. E(2) is second-order stabilization energy [see Eq. (2)], S is overlap integral (see text). In (a), (b) and (d) cutting plane passes through Cl–P–S bonds and in (c) through P–S–C plane and bisector of ClPCl0 angle.

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and donor properties of the molecule. Like in the case of our studies of other phosphites and thiophosphites [7,8,28], we decided to analyze anomeric effects and their influence on internal rotation about P–S bond of Cl2PSMe molecule using NBO analysis where representation of two oxygen or sulfur electron lone pairs in the form of p- and r-type orbitals leads to the better localized Lewis structure [2]. The second-order stabilization energy due to hyperconjugation between 2-center bonding (or 1-center non-bonding lp) and 2-center antibonding orbitals estimated in NBO analysis may be written in the form: ð2Þ

Ei!j  ni

Fig. 4. The barrier to internal rotation about the P–S bond of Cl2PSMe molecule, showing the total energy Etotal (squares), localized Lewis component E(L) (circles), and delocalized non-Lewis component E(NL) (triangles) as function of the dihedral angle s(lpPSC). E(L) reflects steric and electrostatic interactions and E(NL) reflects all hyperconjugations.

We have previously suggested that the equilibrium conformations of mono- and bis- aminophosphanes (A2PNB2 or AP(NB2)2), are stabilized by anomeric effects, i.e. through delocalization of the electron lone pair on N atom into antibonding orbital of rbond of P atom with the most electronegative adjacent substituent [26,27]. Unlike nitrogen, sulfur and oxygen atoms possess two electron lone pairs which are usually represented by two equivalent sp3-orbitals (‘‘rabbit ears” type). However these orbitals remarkably differ in energy and when studying conformations and orbital interactions it is reasonable to consider their representation in the form of one lone pair of p-type, corresponding to pure p-orbital that is perpendicular to the P–S–C plane, and one of rtype, formally sp2 hybridized, lying in this plane. Orbital of p-type lies higher in energy and determines conformational preferences

S2ij j  i

ð2Þ

where: ni orbital population, Sij overlap integral, iðj Þ orbital energy. The barrier to internal rotation about P–S single bond of Cl2PSMe molecule reflects a subtle balance of steric, electrostatic and hyperconjugative factors. The orbital diagrams in Fig. 3 show why such an interactions are strong contributors to conformational preferences. As shown in Fig. 3, each interaction is maximized when the hyperconjugating orbitals are in common plane. This is analogous to p ? p* conjugation of double bonds that is maximized when interacting p-orbitals are coplanar. Fig. 4 shows the torsional behavior for Cl2PSMe molecule as S– Me group is rotated about P–S bond (all other geometrical parameters being allowed to relax to their optimal values along the torsional coordinate). As shown in the heavy solid curve (squares) of Fig. 4 there are two minima: first global minimum corresponding to more stable anti conformer at s(lpPSMe) = 180° and second local minimum corresponding to gauche conformer at s(lpPSMe)  50° lying above in energy. Fig. 4 also contains a dissection of the total energy (Etotal) into Lewis (E(L), orbital populations ni = 2.0) and non-Lewis (E(NL)) parts. The localized Lewis component E(L) corresponds to more than 99.5% of the full electron density, and so nearly exactly incorporates steric and classic electrostatic effects. As shown in Fig. 4, this component predicts local minima at s  70° (global) and s = 180° (local) and maxima at s = 0° and s  150°, that are opposite of full

Fig. 5. Main hyperconjugative stabilizations in Cl2PSMe molecule, showing the torsional dependence of plpS ? rPCl (circles) and plpS ? rPCl0 (triangles). The sum of these two interactions is shown as heavy solid line (squares), that may be compared with the total barrier potential in Fig. 4 and secondary hyperconjugative stabilizations in Fig. 6.

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Fig. 6. Secondary hyperconjugative stabilizations in Cl2PSMe molecule, showing the torsional dependence of lpP ? rPS (circles), rlpS ? rPCl (triangles), and rlpS ? rPCl0 (stars). The sum of these three interactions is shown as heavy solid line (squares), that may be compared with the total barrier potential in Fig. 4 and main hyperconjugative stabilizations in Fig. 5.

potential. That is Cl2PSMe molecule is an example of conformational preferences that are quite unfavorable from a steric or electrostatic viewpoint. In contrast, the non-Lewis component E(NL) exhibits a stronger conformational dependence that compensates the behavior predicted by E(L), leading to minima correctly located nearly s  50° (local) and s = 180° (global). Thus hyperconjugative interactions incorporated in E(NL) clearly provide the surprising stabilization of anti conformer that counter the expected steric and electrostatic effects contained in E(L). Let us examine the hyperconjugative interactions in greater detail. Figs. 5 and 6 illustrate the torsional dependence of five possible lp ? r* interactions together with their sums (heavy solid lines, squares) which determine internal rotation about P–S single bond. It is seen from Fig. 5 that plpS ? rPCl (circles) and plpS ? rPCl0 (triangles) interactions, are largest in magnitude (16 kcal mol1) and exhibit the slope opposite dependence indicating most stabilizations at s  50° and s  145°, respectively, where corresponding orbitals are nearly coplanar. At so doing when plpS ? rPCl orbitals are coplanar at s  50° and their stabilization is maximum in magnitude the plpS ? rPCl0 orbitals are perpendicular and their stabilization is minimum in magnitude and vice versa at s  145°. At s = 180° the plpS ? rPCl and plpS ? rPCl0 interactions are smaller in magnitude (each is  10 kcal mol1) due to smaller overlap (see Fig. 3a and b), but, because there are two such interactions, in sum they give the strongest stabilization ( 20 kcal mol1). It is seen from Fig. 6 that secondary hyperconjugative stabilizations lpP ? rSMe (circles) and rlpS ? rPClðCl0 Þ [triangles (stars)] are

much lower in magnitude (3 and 2 kcal mol–1, respectively). The former has local minimum at s = 0° and global minimum at s = 180° where orbitals are coplanar. The later stabilizations are slope opposite, and when one pair of orbitals is coplanar and gives maximum stabilization the other pair of orbitals is perpendicular and corresponds to minimum stabilization and vice versa. As shown in Figs. 5 and 6, the total stabilization of these five hyperconjugative interactions is strongest at 180° where global minimum of E(NL) and Etotal is found. It is to be noted that E(NL) includes contributions other than the five above mentioned hyperconjugative interactions, however these omitted contributions are much weaker and do not significantly influence conformational angles established by main interactions. The donor–acceptor delocalizations shown in Fig. 3, like in the case of conjugations of double bonds, can influence torsional dependence of geometry of the molecule, occupancies and other descriptors of the interacting NBOs. It is seen from the form of orbitals in Fig. 3, that, to enhance interaction and overlap, the central P–S bond has to become shorter, terminal P–Cl bond has to become longer and ClPS and PSC bond angles have to become larger, what is observed in Table 1 for anti and gauche conformers. For example, in anti conformer P–S bond length is shorter and PSC bond angle is larger than in gauche conformer and in gauche conformer, where plpS ? rPCl hyperconjugation is maximized and plpS ? rPCl0 hyperconjugation is minimized, P–Cl bond is longer than P–Cl0 bond and ClPS bond angle is larger than Cl0 PS bond angle. As indicated by AIM analysis the electron density in P–S bond critical point also is larger for anti conformer (qb = 0.1478 au) than for gauche conformer (qb = 0.1469 au). It is also seen from Fig. 3 that to enhance plpS ? rPCl overlap the amplitude of the backside lobe of acceptor rPCl orbital has to increase what means that polarizations of rPCl antibond and rPCl bond have to increase to P and Cl atoms, respectively. Let us use the example of strongest plpS ? rPCl interaction of Cl2PSMe molecule to illustrate its influence on variations in geometry and NBO descriptors of P–Cl bond when rotating from s(lpPSC) = –50° (gauche–), where the plpS lone pair is perpendicular to P–Cl bond, to s(lpPSC) = 50°, where they are nearly coplanar (gauche+, see Scheme 1 and Table 3). As indicated in Table 3 when rotating from perpendicular to coplanar arrangement of plpS and rPCl orbitals the NBO overlap ð2Þ Splpr , stabilization energy DEplpr and occupancy of rPCl antibond are increased. The charge transfer to P–Cl antibond leads to significantly weakening and lengthening of this bond. The SPCl bond angle remarkably increases leading to additional increase in NBO overlap as compared with rigid rotor model. It is to be noted that populations of donor plpS lone pair orbital are equal for gauche+ and gauche– conformations because in both cases it is coplanar with rPCl or rPCl0 orbitals and interacts equally with rPCl or rPCl0 antibonds, respectively. Small changes in hybridization are also expected to accompany internal rotation. For the acceptor P–Cl antibond at the coplanar

Table 3 ð2Þ Torsional variation of NBO overlap Slpr , stabilization energy DElpr (kcal mol–1), occupancies (e), chlorine charge qCl (e), bond order b, polarization (% on Cl), dipole moment l (D), 0 AIM electron density in bond critical point qb (au), variations in bond lengths Dr (Å A) and bond angles D(SPCl) (°) of Cl2PSMe molecule.

s (lpPSC) –50 –20 20 50

s (plpSPCl) –90 –69 –19 10

plpS ? rPCl

Occupancy

rPCl

qCl

(2)

S

DE

0.010 0.088 0.186 0.213

0.00 3.75 13.36 15.60

Calculated at the HF/AUG-cc-PVTZ level of theory.

0.0509 0.0604 0.0843 0.0906

0.304 0.307 0.324 0.337

P–Cl

D (SPCl)

% on Cl

l

b

qb

Dr

68.4 68.4 68.9 69.4

2.288 2.305 2.401 2.474

0.9973 0.9956 0.9848 0.9828

0.130 0.129 0.127 0.125

0.000 0.004 0.015 0.021

0.0 2.1 6.0 6.2

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arrangement the amplitude of the backside lobe of rPCl is increased by the higher p-character in the hP hybrid to Cl atom that also leads to the lengthening of P–Cl bond, due to extended range of p-orbitals, and weakening of the hP electronegativity, due to higher energy of p-orbitals. At so doing the increase of the s-character in the hP hybrids to other atoms is observed. As shown in the Table 3 the changes in the form of NBO orbital affecting the polarity of the P–Cl bond, its dipole moment, charge on Cl atom etc are also observed during rotation from perpendicular to coplanar arrangement of plpS and rPCl orbitals. Acknowledgements We are grateful to the DFG (project 436 RUS/113/69/0-7) and RFBR (projects 09-03-91340a and 10-03-00320a) for financial support. References [1] J. Vogt, N. Vogt, R. Kramer, J. Chem. Inf. Comput. Sci. 43 (2003) 357. [2] F. Weinhold, C.R. Landis, Chem. Educ. Res. Prac. Eur. 2 (2001) 91. [3] F. Weinhold, C.R. Landis, Valency and Bonding. A Natural Bond Orbital Donor– acceptor Perspective, Cambridge University Press, Cambridge, UK and New York, 2005. [4] M. Korn, H. Oberhammer, R. Minkwitz, J. Mol. Struct. 300 (1993) 61. [5] E.R. Cohen, T. Cvitas, J.G. Frey, B. Holström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H.L. Strauss, M. Takami, A.J. Thor, Quantities, Units and Symbols in Physical Chemistry, third ed., Royal Society of Chemistry, London, 2007. [6] L.C. Cross, W. Klyne, Pure Appl. Chem. 45 (1976) 11. [7] A.V. Belyakov, A.N. Khramov, P.E. Baskakova, V.A. Naumov, Russ. J. Gen. Chem. 75 (2005) 700. [8] A.V. Belyakov, A.N. Khramov, V.A. Naumov, J. Mol. Struct. 698 (2004) 59. [9] J.R. Durig, D.A. Barron, J.F. Sullivan, D.G. Anderson, S. Cradock, D.W.H. Rankin, J. Mol. Struct. 268 (1992) 143. [10] J.R. Durig, J. Xiao, J. Mol. Struct. 526 (2000) 373. [11] D.E.J. Arnold, G. Gundersen, D.W.H. Rankin, H.E. Robertson, J. Chem. Soc., Dalton Trans. (1983) 1989.

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