Structure and bonding in simple thiazyl compounds

Chemical Physics 108 (1986) 417-428 North-Holland, Amsterdam

STBUcIzTBE

AND BONDING

Claus EHRHARDT

417

IN SIMPLE THIAZYL COMPOUNDS

’ and Reinhart AHLRICHS

Institut fii Physikalische Chemie und Elektrochemie, Kaiserstrasse 12, 7500 Karlsruhe, FRG

Lehrstuhl fir

Theoretische Chemie, Universitiit Karlsruhe,

Received 24 February 1986; in final form 27 June 1986

The molecules NSF, NSO-, NSCl, HNSF+, HNSO, CHsNSF+, CHsNSO are investigated at the SCF level and partly with the inclusion of electron correlation effects. Equilibrium geometries and relative stabilities of isomers and conformers are reported. Structure constants and relative energies are discussed in connection with the molecular electronic structure as characterized by the results of population analyses.

1. Introdwtion

Thiazylfluoride (NSF) and thiazyltrifluoride (NSF,) are the two fundamental compounds on which the so-called N-S-F chemistry is based [l]. The molecular structure of NSF has been determined by microwave spectroscopy [2,3] and ab initio investigations [4-71; the results agree within the errors of the respective methods. The theoretical treatments allow for a discussion of electronic structure and bonding in NSF [5-71 and NSF, [7] in connection with chemical properties. Replacement of the fluorine atom in NSF by chlorine leads to the thiazylchloride (NSCl) which has the same structure [8,9] as NSF. The thiazyloxide anion (NSO-) is isoelectronic to NSF. It has recently been prepared in the gas phase reaction: H,N- + SO2 + NSO- + H,O [lo]. The NSO group is present in many compounds of which the organic sulfinylamines ,RNSO are the most important ones [ll]. For the isomeric anion ONSthere is so far only one experimental hint. Tiwari et al. [12] have proposed that the ONS group is bonded via nitrogen in the complex Co(NS0)

WWW,l,. Thiazylimid (HNSO) and methylthionylamine (CH,NSO) are two well-known compounds belonging to the sulfinylamines. The planar cis con’ Present address: Sandoz Ltd., Basel, Switzerland.

formation of HNSO has been established by microwave spectroscopy [13]. The isomeric NSOH can be prepared from HNSO by photolysis [14]. CH,NSO is the most simple member of the organic sulfinylamines [15]. From microwave spectra [16] and the combination of vibrational studies and electron diffraction experiments [17] the cis conformation with eclipsed CH, group (see fig. la below) could be inferred. Apart from calculations of the electric moments of HNSO [l&19] and CH,NSO [19] and investigations of the molecular structure and the force field of HNSO [20] no ab initio calculations of the abovementioned molecules have been published. In this article we report results of electronic calculations for the systems structure NSO-/ONS-, NSCl, HNSO, NSOH, CH,NSO and NSHF+. After a discussion of the relative stabilities of NSO- and ONS- we compare the bonding in NSO- and NSCl with that in NSF. In a previous paper [7], the systems HNSF+ and CH3NSF+ were treated within the simplifying assumption of a linear arrangement H-N-S and C-N-S. In this paper we extend these investigations to non-linear adducts of H+ and CH: with NSF and compare the results with those for HNSO and CH,NSO and discuss the changes of NS and SF (SO) bonds and the relative stability of different isomers.

0301-0104/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

418

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

2. Methods of computations

Table 1 Exponents

of polarization

functions

Most of the geometry calculations in this work were done at the SCF level with the aid of an analytical gradient program [21]. For four of the smaller molecules, NSF, NSCl, NSO- and HNSO, effects of electron correlation were taken into account by means of the CPF method [22]. This method includes all single and double replacements from a single reference configuration and accounts for higher excitations in an approximate way in order to achieve size consistency. All valence MOs were correlated. Electronic structure, electron distributions and bonding are visualized by means of contour diagrams and especially by the results of a population analysis. We applied a recently developed version [23,24] of the population analysis based on occupation numbers of modified atomic orbitals (MAOs) [25]. This analysis, originally proposed by Davidson [26] and Roby [27], characterizes bonding and electron distribution (on the SCF level) by means of atomic charges, QA, and shared electron numbers (SEN) a,,. The SEN have been shown to be a reliable measure of covalent bond strength. Some typical results are: strong covalent single bonds: SEN = 1.4

Atom

Polarization functions

Exponent

H

1P

in CH-compounds: else: 1.00 1.20 0.80 1.00 1.20 1.25 1.50 1.40 1.70 0.55 0.32/0.95 0.66 0.65 0.38/1.13 0.78

(Hz, C-C, C-H), polar single bonds: SEN = 1.13 (HF, Qr = -0.25) to SEN = 0.33 (NaF, Qr = -0.82) strong covalent double bonds: SEN = 2.2 (C=C), polar double bonds: SEN = 1.96 (H,C=O, Q, = - 0.30), triple bonds: SEN = 2.94 (N2). Hypervalent contributions to chemical bonds are discussed in terms of the so-called hypervalency populations [28,23] which range from = 0 in normal valent compounds up to 0.2 in molecules like SF, or ClO; . The Mull&en population analysis [29] was used to discuss effects of changes in hybridization. All computations were performed with the Karlsruhe version [30] of the COLUMBUS system of programs [31-341. All molecules considered in this work have equilibrium structures showing C, symmetry. The following MOs are then occupied in the electronic ground states: 13a’, 3a” for NSF, NSO-, HNSF+, HNSO and 16a’, 4a” for NSCl, CH,NSF+ and CH,NSO, which always leads to a ‘A’ ground state.

H: (5,1)/[3,11, C, N, 0, F: (9,5,1)/[5,3,11, S, Cl: (11,7,1)/[6,4,1]. Most of the CPF calculations were performed with a basis set including up to f-functions: Basis B

C N 0 F S

Cl

Id Id Id If Id If Id If Id 2d If Id 2d If

0.75,

Basis sets For SCF calculations we mainly used the following CGTO basis sets with primitive GTOs from Huzinaga’s tables [35]: Basis A

I-I: (5JJM3JJ1, N, 0, F: (9,5,UM5,3JJl, S, Cl: (11,7,2,1)/[6,4,2,1]. The exponents of the polarization functions are given in table 1. Basis sets which differ from the two standard sets A and B are described in the text.

3. Results of computations and discussion 3.1. Comparison and ONS -

of the two isomeric anions NSO -

The computed structure parameters and relative energies of NSO- and ON% as obtained on

419

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

chemical intuition, that NSO- is more stable than ONS-, and that extended basis sets - including (ldlf) for N, 0 and (2dlf) for S - are required for a reliable treatment. The energy difference between NSO- and ONS- changes considerably from basis A to B*, by = 80 kJ/mol. Further calculations were performed to find out which part of ‘the extended basis set is responsible for this change. As correlation effects have a virtually negligible influence on the relative stability - SCF and CPF results for AE differ by only 4.7 kJ/mol for basis B* - these calculations were done on the SCF level. The extension from basis A to B* was done in three steps in using the geometry obtained for basis A. (i) Extension of the valence basis for 0 and N, i.e. from (9,5,1)/[5,3,1] to (10,6,1)/[6,4,1]. The improved valence basis stabilizes NSO-, by = 40 kJ/mol relative to ONS-, and NSO- is 24 kJ/mol lower in energy than ONS-. (ii) The addition of the second d set on S further stabilizes NSO- by 22 kJ/mol which is

various levels are collected in table 2. Since anions have a more diffuse electron distribution than neutral systems we have employed the extended basis set A(p), which includes additional diffuse p functions on all atoms and slightly more diffuse d functions on 0, N (as specified in table 2). The effect of the diffuse functions is not very pronounced, see lines l-4 of table 2. The computed equilibrium distances differ by at most 0.7 pm for NS in ONS-. For both basis set, A and A(p), NSO- is predicted to be less stable than ONS-, by 15.4 and 19.8 kJ/mol, respectively. In order to be on the safe side we employed the slightly extended basis B* (as compared to B) for the more accurate calculations. Basis B* includes a larger (lOs, 6p) basis for 0, N and slightly more diffuse polarization functions than B, as specified in table 2, compare also table 1. In the calculation with basis B* we employed the geometries obtained on the SCF(A) level. NSO- is now more stable than ONS- on the SCF and on the CPF level, by 65.2 and 69.9 kJ/mol. This result first of all shows, in agreement with

Table 2 Equilibrium geometries and relative stabilities of the anions NSO

and ONS-

computed on the SCF and CPF level a’

Method

Basis

d NS

dso

NSOONS NSOONS NSOONSNSOONS

SCF SCF SCF SCF SCF SCF CPF CPF

A A A (~1 A (~1 B* B* B* B*

146.7 170.6 147.1 171.3 146.7 170.6 146.7 170.6

149.5

basis sets: A:

see text

A (~1:

B*:

b’ b’ b.c’ b.c’

NQ: (9,5,1)/[5,3,11, + lp-set 9, = 0.050.0.065,

7Jd= 0.80,1.00

S: (11,7,1)/[6A,ll, + lp-set ~)a= 0.050 ND: (10,6,1,1)/[6,4,1,11,

qd = 0.55,

S:

(11,7,2,1)/[6,4,2,11,

d ON 119.4

149.7 119.5 149.5 119.4 149.5 119.4

Angle

AE

121.4 118.R 121.0 118.1 121.4 118.8 121.4 118.8

15.4 0.0 19.8 0.0 0.0 65.2 0.0 69.9

qd = 0.90, 1.15, nr = 1.10, 1.40 vd = 0.32/0.95 l)r - 0.66

a) Bond distances d in pm, angles in degree, energy values A E in kJ/mol relative to the most stable structure with A E = 0.0 kJ/mol. b’ Geometry was not optimized but was taken from calculation SCF/A. ” Only valence-shell orbitals were correlated.

420

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

now 46 kJ/mol lower in energy than ONS-. (iii) The final step from A to B*, the addition of f functions for all atoms, stabilizes NSO- by further 19 kJ/mol resulting in the final AE(B*) of 65 kJ/mol, table 2. Since basis set extensions beyond A have a pronounced influence on the computed energetic order of NSO- and ONS-, we have further repeated the geometry optimization for the basis set of step (ii) above, i.e. basis B* but without f functions. The following results were obtained. NSO-: NS = 145.4 pm, SO = 147.9 pm, LNSO = 122O. ONS: NS = 169.7 pm, ON = 119.7 pm, LONS = 118.6”. As compared to the results obtained with basis A, listed in table 2, we find only minor changes. The largest difference occurs for the SO distance in NSO- which is now shortened by 1.6 pm. The geometry optimization had a negligible effect on the SCF energy which was lowered by less than 2 kJ/mol as compared to geometry A, table 2. The foregoing discussion shows that basis A, which is of better than DZP quality, is sufficiently flexible to yield equilibrium geometries close to the SCF limit, experimental results are not known. A reliable computation of the energy difference between NSO- and ONS requires a basis set extension in the valence and in the polarization set both of which stabilize NSO- by = 40 kJ/mol relative to ONS. The electronic structure of NSO- is best rationalized by a comparison with the isoelectronic molecule SO,. SO, shows SO u bonds and a four-electron three-center n system. The latter leads to an accumulation of electronic charge at the ligands. This explains that SO, and comparable eighteen valence-electron molecules are most stable if the ligands are more electronegative than the central atom, compare NSF, OPF, OPCl [36], ONF [37], etc. A further stabilization occurs through stabilizing central atom 3d contribution to the n type HOMO. This reasoning is in line with the Rundle model [38] of four-electron three-center bonding. In NSO- one expects a shift of electronic charge from the N to the 0 position which strengthens the NS bond and weakens the SO bond (as com-

pared to SO,). This is confirmed by the results of the population analysis shown in table 3. The results for NSO- indicate a strong polar NS double bond and a very strong SO single bond, which are stronger and slightly weaker than the SO bonds in SO,, respectively. The hypervalency population of sulfur, us, which is a measure of the 3d(S) contribution, is of comparable size in SO, and NSO-, see table 3. More information is obtained from an inspection of highest MOs in NSO- and ONS. The corresponding a’ HOMO is delocalized and antibonding, the orbital energies (NSO-: e(13a’) = -0.1335 au, ONS: c(13a’) = -0.0979 au) also indicate ONS- to be less stable. More instructive is a consideration of the 2a” and 3a” MOs corresponding to the bonding 2b, and the non-bonding la, of SO,. In NSO- the 2a” MO, c(2a”) = -0.3358, is OS bonding and weakly SN bonding. The 3a” MO of NSO-, ((3a”) = -0.1565 au, is SN bonding and OS antibonding. The net effect of the two valence a” MOs is an NS 71 bond but virtually no (at most a very weak) OS 71 bond. This can be expressed as N-=S+-Oas a shorthand characterization of the electronic structure of NSO- which is in perfect agreement with the results of the population analysis, table 3, and shows the pronounced Coulomb stabilization of both bonds. The two important stabilizing effects - most electronegative atoms as ligands and central atom 3d contribution to the T-type HOMO - are not present in ONS. The corresponding 2a” MO, c(2a”) = -0.3787 au, is almost perfectly localized in the ON bond. It appears that the sulfur 3pa A0 is too high in energy to make substantial contributions. The 3a” MO, e(3a”) = -0.1140 au, is dominantly sulfur 3pa with some contribution from 0 but virtually none from the central atom N. The electronic structure of ONS- is best characterized as O=N-S-, in fair agreement with the results of the population analysis, table 3, which indicates virtually no Coulomb stabilization effects. The foregoing discussion also allows for a rationalization of our finding that the NS and SO bond distance in NSO- and the ON distance in ONS are close to those found in the correspond-

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

Table 3 Computed equilibrium geometries and results of the population comparison with experimental data ‘)

analysis for the molecules

Molecule/method

d NS

dsx

Angle

QN

NSF/SCF NSF/CPF

143.6 145.9

162.6 164.7

113.1 117.0

- 0.42 -

NSF/exp. ‘) NSCl/SCF NSCI/CPF b, NSCl/exp. d,

144.8 144.4 148.8 145.0

164.3 214.0 219.0 216.1

116.9 114.4 117.7 117.7

NSO-/SCF ONS-/SCF SO, /SCF

146.7 170.6

149.5 119.4 142.1

121.4 118.8 117.8

b,

‘)

NSF, NSCl, NSO-,

ONS-

and SO, in

Qx

eNS

%X

us

0.93

-0.52 _

2.19

0.53

0.056

- 0.37 -

0.73 _

- 0.36 _ _

2.14 _ _

0.60

0.048 _ _

- 0.93 0.02 -

0.93 - 0.60 1.28

-1.00 - 0.42 -0.64

1.92 1.17 -

1.15 1.71 1.50

0.090 0.005 0.103

OS

‘) Bond distances in pm, angles in degree. Q: atomic charge, 0: shared electron number (SEN), sets used: SCF: basis A (see text), CPF: basis B (see text). b, Only valence-shell orbitals were correlated. The NSF and NSCl angles were not optimized. ‘) Ref. [37]. d, Ref. [8]. ‘) Quantities referring to the ON bond are given in the columns “SX”.

ing diatomics whereas the NS distance in ONS (170.6 pm, table 2) is much larger than that in NS (149.5 pm). The proposal of Tiwari et al. [12] that ONS- is coordinated via nitrogen in the complex Co (NSO)Cl,[P(OH),], is hard to reconcile with the results of the population analysis. If this group is present in the cobalt complex then a coordination via sulfur or oxygen would be more probable as a consequence of the charge distribution. 3.2. Comparison

421

of NSCI and NSO - with NSF

The calculated equilibrium geometries of NSCl, NSO- and NSF are collected in table 3 together with experimental values. The results of the SCF calculations with basis A are as expected. A set of polarization functions on every atom’ tends to yield bond distances = l-2 pm shorter than experiment on the SCF level. Methods including electron correlation, in a size consistent way (like CPF), normally yield bond distances which are up to 2 pm (Cl, [39]) too large for (ldlf) or (2dlf) polarization sets [39-421. The CPF results for NSF confirm this experience, see table 3. An unusually large discrepancy occurs for NSCl, where the CPF results for basis B are 3.8 pm (NS) and 2.9 pm (SCl) larger than experiment [8]. We see no reason to question the equilibrium distances obtained independently by microwave

a: hypervalency

population.

Basis

[8] and electron diffraction methods [9]. Since g functions influence R, especially for second-row diatom& e.g. a shortening by 1 pm for Cl, [39], we have performed additional calculations. These were carried out with a (2dlf) basis for N and a (2dlflg) basis for S and Cl. The NSCl angle was kept fixed at 117.7”. Within the CPF method we then obtained R,(NS) = 148.7 pm and R,(SCl) = 216.8 pm. The SC1 distance now overshoots experiment by only 0.7 pm but the NS distance is still 3.7 pm too large. The basis set used here is clearly somewhat unbalanced, one g A0 on S and Cl but not on N. A further basis set increase (which we could not afford) will probably reduce the NS distance by = 1 pm. That would still leave us with a discrepancy of = 2.5 pm to experiment. This is the largest deviation encountered so far in applications of the CPF method [39-421, for which we have no convincing explanation. The SC1 and SF bonds in NSCl and NSF are relatively weak polar (Qr = -0.52, Q,, = -0.36) single bonds, a,, = 0.53 and us,-, = 0.60, table 3. The SO bond in NSO- is a very strong polar (Q, = - 1.0) single bond with some double-bond character as is indicated by uos = 1.15. This rather large u indicates back transfer of electrons from the rather diffuse 2p A0 - note that Q, = - 1.0 into low-lying AOs of sulfur. This reasoning is in line with the pronounced hypervalency population us = 0.09, which is larger than that of NSF, us =

422

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

0.056, and comparable to that of SO,, us = 0.10. The variation of the NS bond in the three molecules is of more interest. For NSF, Zirz and Ahlrichs [7] proposed a covalent bond order of - 2.5 plus additional ionic contributions, and Seeger et al. [6] estimated a value of 2.8 for the total bond order. These numbers are in line with the results of the present population analysis (table 3). Going from .NSF to NSCl the covalent bond strength remains almost the same while the charge on the sulfur atom decreases from +0.93 in NSF to 0.73 in NSCl due to the lower electronegativity of chlorine as compared to fluorine. This effect implies weaker ionic contributions and is mainly responsible for the longer NS distance in NSCl, as compared to NSF. Because of the negative charge of NSO- one has to be careful in comparing the results of the population analysis with those of the two other molecules. This point will be discussed in some detail in connection with the comparison of HNSF+ with HNSO, see below. Anions have a more diffuse electron distribution so that the bond lengths are expected to be slightly larger than in analogous neutral molecules. This can be looked upon as one reason for the longer NS distance in NSO-. While u,.,s in NSO- is smaller than in NSF and NSCl, the strong ionic contributions should increase the NS bond strength to a considerable amount and effect a bond shortening. So the calculated bond length is somewhat shorter than one would expect from the results for the SEN alone. 3.3. N-adducts CH,”

of NSO - and NSF

with H i and

In this section we first compare the computed structure constants of HNSO and CH,NSO with available experimental data, mainly to check the accuracy of the method of computation. Next we discuss the relative stabilities of the linear HNS arrangement in comparison with the cis and trans forms for HNSO and HNSF+. These considerations are then extended to CH3NSF+. Bonding of H+ or CHT to NSO- or NSF leads to pronounced changes in the electronic structure which, furthermore, depends on the molecular structure. This

will be discussed in detail for the cis and the linear forms. We finally deal with the relative stability of HNSO and NSOH. 3.3.1. Structure of HNSO and CH,NSO The bond lengths of cis HNSO as calculated on the SCF level are 1-2 pm shorter than the measured values as is expected for basis A. On the CPF level the calculated bond distances are = 1 pm too long (table 4). Deviations of this order and sign are typical for CPF results with the basis sets used [39-421. They are also within the experimental errors [13]. The geometry of cis HNSO has also been investigated theoretically by Raghavachari [20]. In the most sophisticated calculation, inclusion of electron correlation on the MP3 level, this author obtained a very good agreement with experiment (deviations of at most 1.7 pm in distances and 6.3” in the HNS angle). We consider this close agreement to be fortuitous since a small basis set - polarization functions only for sulfur has been used. A possible reason could be an error cancellation of basis set deficiencies with higher order terms of the Moller-Plesset expansion. For the geometry optimization of CH,NSO (table 5) we only considered the experimentally determined cis configuration with an eclipsed CH, group (fig. la) [16,17]. The SCF results obtained in using basis A for interatomic distances compare favourably with the older experimental data of Rao et al. [16], table 5. The computed distances are 2: 2 pm shorter than experiment as is expected on this level of theory. The agreement of the present data with the more recent measurements of Beagly et al. [17] is unsatisfactory, especially for the CN distance of 142.1 [17] versus 145.2 pm (present), table 5. This discrepancy of 3.1 pm could be due to a failure of the SCF method in this case, which is conceivable since the NS and CN distances are quite strongly coupled. However, it is quite unusual that the SCF method overshoots equilibrium distances if the basis includes polarization functions, like basis A. The structure determination by means of electron diffraction measurement (in connection with IR spectra) [17] is in fact difficult in this case. The radial distribution curve shows a major structureless peak around 147 pm which is due to the CN,

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

423

Table 4 Computed equilibrium geometries and relative stabilities of cis, tram and linear HNSO and HNSF+ and cis and trans NSOH ‘) Molecule

Method

kt,ou

d NS

d,

L(HNS)/(SOH)

,!(NSO)/(NSF)

cis-HNSO cis-HNSO cis-HNSO tram-HNSO linear-HNSO cis-NSOH cis-NSOH trans-NSOH cis-HNSF+ trans-HNSF+ linear-HNSFi

SCF CPF b’ exp. ‘) SCF SCF SCF exp. d, SCF SCF SCF SCF

100.5 101.9 102.9 100.4 98.6 94.8 96 94.6 101.3 101.4 100.4

149.4 151.9 151.2 150.0 145.4 145.0 146 144.7 144.7 146.5 141.3

143.9 145.8 145.1 143.2 143.5 163.5 162 164.7 153.4 152.0 153.5

117.5 115.0 115.8 109.6 180.0 112.9 100 112.1 132.1 120.8 180.00

118.5 120.4 120.4 114.8 119.5 114.0 115 111.5 111.3 104.5 111.1

AE 0.0

21.0 73.5 42.0 _ 60.4 0.0 17.3 28.1

a) Bond distances in pm, angle in degree, energy values AE in kJ/mol relative to the most stable structure with AE = 0.0 kJ/mol. Basis sets used: SCF: basis A (see text), CPF: basis B (see text). b, Only valence-shell orbitals were correlated. The NSO angle was not optimized. ‘) Ref. [13]. d, Ref. [14] (estimated values).

SO and NS distances. The peaks for NO and CS also overlap into a single one at 259 pm. These quite unfortunate circumstances should cause appreciable uncertainties of the structure constants determined from the measurements. 3.3.2.

Relative

HNSO

stability

and HNSF

of cis,

tram

and

linear

+

A model type investigation of HNSF+ has already been published, where a linear HNS arrangement was assumed since a thorough investigation was not feasible [7]. In addition to the linear structures we now consider also the cis and trans forms of HNSO and HNSF+. The geometry for each structure has been optimized, and the results and the relative energy values are collected

in table 4. In both cases the cis form is the most stable one. The linear structure is not a local minimum and shows the highest total energy. This ordering is in agreement with experimental results for HNSO which show the cis isomer to be most stable [13]. The linear forms have the shortest NS distances and the larges SEN uNs, table 4, as compared to cis or trans forms. This gain in bond energy is apparently insufficient to compensate for the larger promotion energy required for the sp hybridization of nitrogen. The Mulliken population analysis shows in fact the larger nitrogen 2p gross occupation for the linear cases, table 6. Since XNSO and XNSF,,, compounds generally prefer a cis structure [l] we now want to

Table 5 Equilibrium geometries and relative stabilities of cis, tram and linear CHsNSF+

and cis CH,NSO

computed on the SCF level a)

Structure

d CN

d NS

d sx

L(CNS)

-WW/(NW

AE

c-CH,NSF+ t-CH,NSF+ I-CHsNSF+ c-CH,NSO exp. b, exp. ‘)

146.4 146.8 145.7 145.2 142.1 147

145.4 144.5 141.9 149.3 152.5 151

155.0 154.0 155.3 144.6 146.6 145

139.8 144.8 180.00 125.4 126 122

110.8 106.2 111.1 118.2 117 121

0.0 29.7 23.7

‘) Bond distances in pm, angles in degree, energy values AE in kJ/mol relative to the most stable structure with AE = 0.0 kJ/mol. Basis A (see text) was used for all computations. c, t and I stand for cis, tram and linear CNS arrangement. b, Ref. [17]. ‘) Ref. [16].

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

424

discuss this effect in detail for the molecules HNSO and HNSF+. The higher stability of the cis forms, as compared to trans, is mainly due to the greater strength of the NS bond as can be seen from the calculated NS distances, table 4, and the corresponding SENs, table 6. A detailed investigation reveals that this effect is related to the properties of the nitrogen lone-pair orbital. The corresponding orbitals, as obtained from a Boys localization procedure, are depicted in fig. 2 for HNSF+ as an example. The nitrogen lone pair is involved in a 2p3d backbonding from nitrogen to sulfur. This bonding is more efficient in the cis isomer because the larger lobe of the nitrogen lone pair is in trans position to the SF(S0) bond so that the backbonding does not interfere with the SF(O) bond. To get a numerical value for this effect we repeated the population analysis using LMO vectors instead of MO vectors as input and looked at the contributions of the individual LMOs to the SENs. For the NS bond the main contribution comes from the two LMOs forming the double bond. In the case of the cis isomers there is an additional bonding component originating from the abovementioned lone pair of 0.15 for HNSF+ and 0.05 for HNSO. The same LMO is of much less impor-

H

2 C Q

H

N

S

0

d b

Jr H

H

R

C

H

H

C

ws

N

Table 6 Results of the population Molecule

N

e 0’F

8

Fig. 1. Representation of molecular geometries. (a) CHsNSO (cis, eclipsed). (b) CH,NSF+ (tram, staggered), (c) CHsNSF* (trans, eclipsed), (d) CHsNSF+ (linear eclipsed), (e) CHsNSF+ (cis, eclipsed).

s

analysis for cis, tram and linear HNSO and HNSF+ C)

and for cis and tram NSOH a) N

N

ONS

%o

US

“,

1.31 1.30 1.32 1.22

1.68 1.64 1.74 2.08

1.44 1.46 1.42 0.80

0.076 0.082 0.094 0.056

3.64 3.66 3.49 _

3.89 3.83 4.17

1.22 1.26 1.25 1.25

2.08 1.99 1.90 2.13

0.76 0.86 0.91 0.77

0.057 0.046 0.045 0.061

3.62 3.65 3.52

3.65 3.59 3.82

QN -0.56 - 0.53 - 0.66 - 0.53

Qs 1.14 1.11 1.21 0.88

Q, b’

aYH

c-HNSO t-HNSO I-HNSO c-NSOH

QH 0.13 0.12 0116 0.19

-0.71 -0.68 - 0.71 - 0.54

t-NSOH c-HNSF+ t-HNSF+ I-HNSF+

0.18 0.21 0.21 0.23

- 0.48 -0.16 - 0.15 - 0.21

0.85 1.24 1.21 1.31

- 0.53 -0.29 - 0.25 -0.33

*) Q: atomic charge, e: shared electron number (SEN), u: hypervalency population, nN: Mulliken occupation The abbreviations c, t, I stand for cis, trans. linear. Basis set A (see text) was used for all calculations. “X=O,F. @Y-N,O.

“P

number for nitrogen.

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

la

425

the positively charged hydrogen and the negative fluorine (oxygen) may be looked upon as a further stabilizating effect occurring in the cis structures. In the case of cis HNSO there are also experimental hints for this attraction [14]. However, the cis structure is also preferred if electrostatic interactions favour a tram position, e.g. in CF,NSF, [l]. 3.3.3. Relative

stability of cis, tram and linear CH, NSF + The structures considered are shown in fig. 1.

Fig. 2. Contour diagrams of localized MOs of HNSF+ which correspond to the (o type) nitrogen lone pairs. Lines plotted: 0 (CrOSSed), *0.02, 410.05, f0.125, *0.30, ItO.75. (a) cis HNSF+, (b) tram HNSF+.

tance in the case of the tram isomers (0.05 for HNSF+, -0.0007 for HNSO). The difference between the bonding contribution of the lone pair in the cis and tram forms is approximately the same as the difference between the SENs in both kinds of isomers (see table 6). The pi backbonding from nitrogen transfers charge into empty orbitals on sulfur which are of SF(S0) antibonding type. This idea is in agreement with the fact that the SF(S0) bond is somewhat longer in the cis forms compared to the tram forms (see table 4). The attractive electrostatic interaction between

Only the conformation with an eclipsed CH, group was considered for the cis and the linear isomers, since this is most stable for the isoelectronic CH,NSO [16,17]. For the trans form both conformers, staggered and eclipsed, have been investigated. The staggered form turned out to be less stable by 4 kJ/mol and will not be considered further. The structure parameters and relative energies are reported in table 5. As for HNSF+ we find the cis form of CH,NSF+ to be the most stable one. Our discussion of the stability of isomers of HNSF+ holds in principle for CHJNSF+, too (compare tables 6 and 7). There is a difficulty, however. The CNS angle in cis-CHsNSF+, 139.V, table 5, is larger than the HNS angle in HNSF+, 132.1”, table 4. This is expected as a consequence of steric effects. The corresponding difference in angles is much more pronounced in the tram isomers, LHNS = 120.8“ for HNSF+ versus LCNS = 144.8O in CH,NSF+. Tram CH,NSF+ is much closer to the linear case than is found for HNSF+. The only explanation we can offer is that steric effects as well as Coulomb repulsions cause this unusual bond angle (as compared to tram I-INSF+). As a consequence the trans conformer of CH,NSF+ is even higher in energy than the linear one and shows only a not well pronounced local minimum. Tram CH,NSF+ is relatively close to the linear form which allows us to rationalize the longer NS distance in the more stable cis form. 3.3.4. Relative stability of HNSO and NSOH Photolysis of cis HNSO leads among other molecules to cis NSOH. The equilibrium geometry of cis NSOH has been estimated by Tchir and Spratley [14], and their results are nicely con-

C. Ehrhardt, R. Ahlrichs / Structure and bonding in thiazyl compounds

426

Table 7 Results of the population

analysis

for cis, tram and linear CHsNSF+

and for cis CH,NSO

Structure

Qc

QN

Qs

Qx b,

%N

ONS

%X

c-CH,NSF+ t-CH,NSF+ /-CHsNSF+ c-CH,NSO

0.06 0.09 0.07 0.01

-0.14 -0.17 -0.16 - 0.50

1.17 1.17 1.22 1.11

-0.32 -0.31 -0.37 -0.74

1.21 1.20 1.20 1.29

1.94 1.95 2.10 1.68

0.83 0.83 0.73 1.42

a) Q: atomic charges, e: shared electron number (SEN), u: hypervalency c, t and I stand for cis, tram and linear CNS arrangement. ‘) X = F, 0.

firmed by the present calculations except for the .&OH, see table 4. Again the cis isomer is more stable than trans NSOH. While the delocalization of the nitrogen lone pair increases the bond strength for the NS bond in HNSO or HNSF+, the lone pairs on the oxygen atom cause an additional bonding contribution to the SO bond in NSOH. This backbonding is again more pronounced for the cis form so that the SO bond is shorter in this isomer (d,, = 163.5 pm, use = 0.80) than in the trans structure (d,, = 164.7 pm, us0 = 0.76, see tables 4 and 6). This fact and the additional attracting N-H interaction of Coulomb type are the reasons for the higher stability of the cis form. The formation of the OH bond in NSOH weakens the SO bond, which is a (weak) polar single bond (us0 = 0.80/0.76 cis/ trans). Due to the fact that sulfur is less involved in the SO bond in NSOH (as compared to HNSO) the NS bond is even shorter than in the linear form of HNSO (see table 4). Nevertheless cis/trans-NSOH is less stable than cis/transHNSO. The weak SO bond in NSOH can be looked upon as the main reason for this energy difference. 3.3.5. NSF and NSO - in comparison with their N-adducts with H + and CH,’ In comparing NSF and NSO- with N-adducts we consider the most stable cis and the linear forms, which always have the shortest NS bond length. In compounds with linear acceptor N-S bonds (investigated in this work) the Lewis acid draws off electrons from the rest of the molecule so that especially the sulfur becomes more positive. As a consequence the sulfur orbitals are

population,

a)

b)

nN: Mulliken

N us

ns

0.045 0.048 0.058 0.074

3.62 3.60 3.55 _

occupation

number

N “D

3.58 3.65 3.71

for nitrogen.

lowered energetically and enhance the covalent bond strengths of the NS as well as the SF(S0) bonds, see tables 6 and 7. This results in shorter NS and SF(S0) distances in the adducts as compared to NSF or NSO-. This fact has already been pointed out by Zirz and Ahlrichs [7] and is in agreement with the experimental results for complexes like [Ni(NSF)6]2+ and [Co(NSF),]2’ [l]. The bond shortening (in linear adducts of NSF and NSO- with Lewis acids) is most pronounced for the SF and SO distances, tables 4 and 5. This trend is also reflected by the SEN of SF and SO bonds which increases by = 0.3 in HNSF+ and HNSO as compared to NSF and NSO-. The NS bond is shortened by a lesser extent which is not reflected by an increase in the SEN for the NS bond, compare tables 4, 5 and 6, 7. However, bond dissociation energies (of covalent bonds) correlate with the product of the SEN with the average of atomic ionization potentials [23]. Since ionization potentials are larger for cations and smaller for anions (than for the neutral system), comparable SENs indicate a stronger bond for cations and a weaker one for anions, as compared to neutral systems. In the cis isomers of the N-adducts, hybridization on nitrogen is changed in the direction from sp to sp2 so that the NS distances are now longer than in NSF or NSO- (see tables 4-7). The effects due to the increased positive charge on sulfur then lead to a shortening of the SF(S0) bonds only. Since bonding of a Lewis acid to S in NSF has been briefly discussed [l], we finally investigated NSHF+ with H bonded to S. The energy of this isomer is 336 kJ/mol (on the SCF level basis A) higher than that of cis HNSF+. This result is not

C. Ehrhardt, R Ahlrichs / Structure and bonding in thiazyl compoundr

unexpected since the positive charge of sulfur in NSF, Qs = +0.93, table 3, causes a strong Coulomb repulsion with H+ leading to a very weak SH bond only. A attack of a cationic Lewis acid on S in NSF therefore appears unlikely in general.

4. Summary Molecular geometries computed on the SCF level have been reported which are in close agreement with experimental results for NSF, NSCl and HNSO, e.g. deviations of l-2 pm in interatomic distances. A discrepancy occurs for CH,NSO where the SCF results support the older values of Rao et al. [16] but show unusually large deviations from the more recent results of Beagley et al. [17], section 3.1, which are attributed to problems in the evaluation of experimental data. Treatments which include effects of electron correlation require large basis sets - (2dlf) or even larger - to converge equilibrium distances to 1 pm. For NSCl we get all equilibrium distances too large (3.7 pm for d,,) on the CPF level even if a (2dlflg) polarization basis is employed for S and Cl. The deviation is appreciably larger than found in other applications [39-421. We have no convincing explanation for this discrepancy. It is shown that NSO- is more stable than the isomer ONS- by 70 kJ/mol (on the highest level of theory, CPF, table 2), and HNSO (cis) more stable than NSOH (cis) by 42 kJ/mol, table 4. The adducts XNSF+ and XNSO, X = H, CH,, always prefer a (planar) cis XNSF+, XNSO arrangement, as compared to trans or linear. The crucial factor for the stability of the cis form turns out to be a ‘better delocalization of the nitrogen lone pair into low lying AOs of sulfur, 2p-3d backbonding. For the linear XNS arrangement (in XNSF or XNSO) we always find shorter NS and SF(S0) distances than in NSF(NSO-), whereas in the cis forms only the SF(S0) bond is shortened. The linear XNS moiety requires an sp hybridization of N for which stronger bonds result but a higher promotion energy is required as compared to the sp2 hybridization of the cis forms. The gain in the

427

bond energy does not compensate for the higher promotion energy in this case.

Acknowledgement We thank Dr. Mews, Gbttingen, and Dr. Oberhammer, Tubingen, for useful discussions. This work was made possible by support from the DFG. The authors gratefully acknowledge the excellent service provided by the “Rechenzentrum der Universitat Karlsruhe”.

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