A theoretical study of the structure, electronic properties, and reactivity of phenylcyanates

THEO CHEM ELSEVIER

Journal of Molecular Structure (Theochem) 401 (1997) 69-76

A theoretical study of the structure, electronic properties, and reactivity of phenylcyanates John 0. Morley*, Memdoh Naji Chemistry Department, University of Wales, Swansea. Singleton Park, Swansea SA2 8PP, UK

Received 30 September 1996; accepted I9 December I996

Abstract Calculations are reported on the structure and electronic properties of phenylcyanate and its 4-nitro and 4-amino derivatives using both the semi-empirical AM1 and PM3 methods and the ab initio 6-31G ** basis set. The calculated structures are compared with crystallographic data where available and their electronic properties with their expected reactivity. A study of the reactions of the phenylcyanates with methoxide ion in methanol using the AMIKOSMO method suggests that the ratecontrolling step of the nucleophilic substitution reactions of the phenylcyanates is controlled by the addition of methoxide ion to

the cyanato group to give a stable intermediate. Ring substituents exert little effect on this process, although, thermodynamically, the formation of the 4-nitrophenylcyanate intermediate is favoured over the others. The conversion of the intermediate into products is strongly affected by the nature of the ring substituents so that both the kinetics and thermodynamics of the subsequent displacement of the phenoxide ion is facilitated by the nitro group and retarded by the amino group. 0 1997 Elsevier Science B.V. Keywords:

Phenylcyanates;

Structure; Reactivity;

AMKOSMO

method

1. Introduction Phenylcyanate (la) and many of its derivatives such as (lb) and (lc) are important intermediates for the manufacture of a variety of industrial chemicals which include polymers and insecticides [l-3]. Chemically, they are readily attacked by nucleophiles mainly at the cyano-carbon, leading to stable adducts in some cases or the displacement of phenol in others [1,4-91. (1) la R = R’ = R* = R3 = R4 = H; lb R = RI = R3 = R4 = H; R* = NO,;

* Corresponding author. Ol66-1280/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. P/I SOI 66- 1280(97)00008-O

lc R = R’ = R3 = R4 = H; R* = NH*; ld R = R4 = t-Bu; R’ = R* = R3 = H; le R’ = R3 = Me; R = R4 = H; R* = Cl; If R = R’ = R3 = R4 = H; R* = C(Me)2C6H40CN.

J.O. Morley, M. Naji/Joumal of Molecular Structure (Theochem) 401 (1997) 69-76

70

For example, phenylcyanate (la) reacts with alcohols in the presence of catalysts to give the corresponding phenoxyalkoxyimide (2) in high yield [7,8]. While the imide (2) is relatively stable, it undergoes further reaction with alkali to give an alkylcyanate (3) and phenol (Eq. (1)) [4]. In contrast, 2,6-di-t-butylphenylcyanate (Id) is hydrolysed by aqueous sodium hydroxide to give 2,6-di-r-butylphenol, probably via the intermediate imino anion (4), although the precise mechanism is unknown (Eq. (2), R = H) [9] Ar-O-C-N

+ ROH * Ar-G

C= NH + Ar-OH + R-O-C-N I OR

(I)

(1)

(2)

Ar-O-C=N + RO- *Ar-O-C=

(3)

N-+Ar-0-+

R-O-C=N

I OR (0

(4)

(2) (3)

It is well established experimentally that ring substituents have a large effect on the reactions of related molecules such as benzoyl and benzenesulphonyl halides and esters with a variety of nucleophiles, with electron donors retarding the reaction and electron attractors facilitating the process [lO,ll]. However, the transmission of substituent effects to the reactive cyanato-carbon of the phenylcyanates has received much less attention and it is not known whether the adjacent oxygen atom is able to modify or block this effect. Theoretically, the electronic structures of a number of phenylcyanates containing both donor and acceptor groups have been reported using the STO-3G basis set, but the effect of substituents on the reactive cyanato-carbon were not disclosed [ 121. The studies described here have been carried out to probe the structure and electronic properties of phenylcyanate (la) and to assess the likely changes to the reactivity pattern which would result from the introduction of both an electron-attracting nitro-group and electron-donating amino-group into the phenyl ring to give structures (lb) and (lc), respectively. In addition to this early transition state model (which assumes that the transition state is reactant-like), the nucleophilic reactions of the three phenylcyanates

with the methoxide ion in methanol have been explored along the reaction coordinate using a semiempirical continuum approach.

2. Methods of calculation Calculations were carried out with full geometry optimisation on phenylcyanate (la) and the 4-nitro (lb) and 4-amino (lc) derivatives, using both the semiempirical AM1 [ 131 and PM3 [ 141 methods of the MOPAC program [ 151, and the ab initio 6-3 1G** basis set [ 161 of the GAMESS program [ 171. The numbering convention adopted for the calculations is shown in Scheme 1. Nucleophilic reactions of the phenylcyanates (la), (lb), and (lc) were carried out using the COSMO method [ 181 implemented in the MOPAC [I51 program at the AM1 level [ 131 (keywords: ‘am1 precise xyz saddle eps = 32.6 charge = -1’). The transition state was approached from both the reactants’ side of the reaction, with the methoxide ion placed perpendicularly at a distance of 10 A from the cyanato-carbon at the starting point, and from the products’ side of the reaction, with the displaced phenoxide ion again placed perpendicularly at a distance of 10 A from the same carbon at the end of the reaction. The energy was evaluated as a function of the decreasing distance of the approaching or departing ion from the cyanato-carbon with the geometry fully optimised at each point. Two clearly defined saddle points were found at each side of a

Scheme I. Numbering convention adopted for the phenylcyanate (la), 4-nitrophenylcyanate (lb, X = 0). and 4-aminophenylcyanate (lc, X = H).

J.O. Morley.

M. Naji/Journal

of Molecular

stable intermediate along the reaction coordinate in each case. Both transition states (TSl and TS2) were refined and shown to be first-order saddle points by their unique single imaginary vibrational frequency. For the first transition state, the intrinsic reaction coordinate path (IRC) was followed both forwards to a stationary point for the intermediate and backwards to the reactants. The second transition state was characterised similarly by running the IRC backwards to the intermediate and forward to the products.

3. Results and discussion 3.1. Calculated structure and electronic properties Experimentally, there are few structures available for phenylcyanates present in the Cambridge Structural Database [ 191 for comparative purposes, with the exception of 4-chloro-3,5dimetbylphenylcyanate (le) [20] and 2,2’-bis(4-cyanatophenyl)isopropylidene (If) [21]. In the former (le) [20], there are two molecules in the unit cell which show average bond lengths of 1.42 A (Cl-07) 1.27 A (07-C8), and 1.13 A (C8-N9), average angles of 114” (C2-Cl-07), 118” (Cl-07-C8), and 174” (07-C8-N9), and an average torsion angle of 170” (C2-Cl-07-C8). In the latter (lf) [21], there are two cyanato groups positioned at either end of the molecule which show average bond lengths, in this case of 1.43 A (C l-07), 1.28 A (07-C8), and 1.12 A (C8-N9), and average angles of 115” (C2-Cl-07), 118” (Cl-07-C8), and 176” (07-C8-N9). However, the torsion angles (C2-Cl07-C8) are different at either end of the molecule with values of 171.7 and -156”, respectively. Initial calculations carried out on phenylcyanate (la) using the semi-empirical AM1 [ 131 and PM3 [ 141 methods result in an underestimation of the Cl-07 bond length and overestimation of the 07$8 bond length, in both cases with values of 1.41/l .40 A and 1.34/l .33 A (Table 1) versus expected values of 1.43 and 1.28 A, based on crystallographic data [20,21]. Both methods give a good account of the bond angles at C2-Cl-07 and Cl -07-C8 with values of 114/l 13” and 115/115”, respectively (Table l), versus expected experimental values of 115 and 118”. While the PM3 method places the

Structure

(Theochem) 401 (1997) 69-76

11

cyanato group roughly in the ring plane, the AM1 method twists the group by around 14” to the ring plane (Table 1). Similar bond lengths and angles are calculated at the same positions for 4-nitrophenylcyanate (lb), though the molecule is predicted to be essentially planar with both the nitro and cyanato groups in the ring plane. In contrast, both the AM1 and PM3 methods predict that the substituents in 4aminophenyl-cyanate (lc) are twisted from the ring plane, with the cyanato group 47-53” out of plane and the amino-group in an approximate sp” conformation. Again similar bond lengths and angles are found at the cyanato group (Table 1). The results obtained at the 6-31G** level, however, are generally superior to those obtained at the semiempirical level, except that the 0Cl-07 bond length appears to be shorter at 1.398 A than that expected from crystallographic data at 1.43 A (Table 1). However, the calculated 07-C8 bond length matches the experimental value at 1.280 A, and the calculated angles at C2-C l-07 and C 1-07-C8 show an excellent correlation with the experimental data with values of 115” and 119” versus 115” and 118”, respectively (Table 1). The introduction of a nitro group at the 4position to give structure (lb) results in a shortening of the Cl-07 bond length to 1.385 A and a slight increase in the 07-C8 bond length to 1.284 A. In the 4-amino derivative, the opposite effect is observed with an increase in the Cl -07 bond length to 1.405 A and a slight contraction in the 07-C8 bond length to 1.278 A. At the ab initio level, the cyanato group is predicted to be twisted from the ring plane in all three phenylcyanates (la), (lb), and (lc) by around 18-19”, respectively, in contrast to the planar structures predicted by the semi-empirical methods (Table 1). An analysis of the calculated atomic charges for the phenylcyanates shows that there are significant differences between the semi-empirical and ab initio methods (Table 2). In the former, the electronic differences found among the phenylcyanates (la), (lb), and (lc) at the cyanato group are relatively small, with the critical charge at the cyanato-carbon, C8, of the parent (la) almost identical to that calculated for the 4-amino derivative (lc), with the 4-nitro derivative (lb) unexpectedly more negative at this level of theory. At the ab initio level, the insertion of a nitro group into phenylcyanate to give structure (lb) results in an

72 Table 1 Geometries Geometric

J.O. Morley, M. Naji/Journal of Molecular Siructure (Theochem) 401 (1997) 69-76

of phenylcyanate variablea

(la) and the 4-nitro (lb) and 4-amino derivative

(la) PM3

1.402 1.393 1.396 1.394 1.396 1.409 1.342 1.164

118.3 120.5 120.3 120.4 118.4 122.2 114.1 114.9 174.4

- -166.4

a Bond lengths in angstroms,

at the semi-empirical

(lb)

AM1 Cl-C2 C2-C3 c3-c4 c4-cs C5-C6 Cl -07 07-C8 C8-N9 C4-NIO NIO-XI 1 NlO-X12 Cl -C2-C3 C2-C3-C4 c3-c4-c5 C4-C5-C6 C.5-C6-C 1 C2-Cl-C6 C2-Cl-07 Cl-Ol-C8 07-C8-N9 C3-C4-NIO C4-NlO-XI 1 X1 1-NlO-X12 C2-C l -07-C8 C3-C4-NIO-Xl1

(lc) calculated

1.400 1.389 1.392 1.391 1.391 1.403 1.334 1.162

118.5 120.5 120.3 120.5 118.5 121.8 113.2 115.3 173.6

-177.4

6-31G** 1.379 1.383 1.387 1.384 1.387 1.398 1.280 1.136

118.4 120.5 119.8 120.6 118.2 122.6 115.2 119.1 178.6

-160.7

AM1 1.405 1.389 1.407 1.404 1.393 1.403 1.345 1.163 1.486 1.201 1.201 118.8 119.6 121.1 119.5 118.9 122.1 113.8 115.3 174.1 119.4 118.8 122.3 179.9 179.9

and ab initio levels

w PM3 1.401 1.387 1.402 1.400 1.389 1.396 1.336 1,161 I .499 1.214 1.214 118.9 120.3 120.0 120.3 119.0 121.6 113.3 115.5 173.5 120.0 119.3 121.5 -179.9 -179.9

6-31G** 1.382 I.379 1.385 1.383 1.380 1.385 1.284 1.135 1.456 1.193 1.192 118.8 118.8 122.2 119.2 118.3 122.8 115.0 119.5 178.7 119.1 117.5 125.0 -161.8 179.7

AMI 1.399 1.388 1.417 1.416 1.389 1.411 1.342 1.164 1.395 0.995 0.995 119.1 120.7 118.6 120.9 119.0 121.7 115.0 114.2 174.7 120.6 115.0 113.9 -152.9 - 159.4

PM3 1.398 1.387 1.404 1.404 1.387 1.407 1.334 1.162 1.427 0.995 0.995 118.9 120.2 120.0 120.2 119.0 121.8 114.6 114.0 174.3 119.9 112.0 111.5 -146.3 -155.6

6-3lG** 1.377 1.380 1.397 I.394 1.383 1.405 1.278 1.137 1.372 0.990 0.990 119.5 120.7 118.3 121.1 119.1 121.3 115.9 118.8 179.1 120.9 120.9 117.9 -161.1 179.3

angles in degrees.

increase in the positive charge at ring carbon C 1, and a change in the charge at ring carbon C4 from negative to positive. The amino group exerts a similar but larger effect at ring carbon C4 in structure (lc), but the positive charge at ring carbon Cl is reduced and negative charge at C3 and C5 is increased by classical electron donation from the amino group at the 4-position. Surprisingly, the introduction of the nitro or amino group appears to have little electronic effect on the cyanato group which retains almost the same charge at the C8 atom in all cases, although there are extremely small effects at the 09 and N9 atoms which reflect the change in the nature of the group at the 4position of the respective molecule. The calculated dipole moments of the three phenylcyanates show the same trends at both the semiempirical and ab initio levels, with the value of the parent (la) increasing with the introduction of the electron-donating amino-group to give structure (lc)

and reducing with the introduction of the nitro group to give structure (lb) (Table 2). In the last case, the presence of two electron attractors situated at either end of the aromatic ring results in a fairly small experimental dipole moment of 2.27 D [22] which is well reproduced by the 6-3 lG** calculation at 2.30 D (Table 2). However, the corresponding calculated value for the parent (la) at 4.79 D overestimates the experimental value of 3.93 D [22] for reasons which are not entirely clear. While neither of the semi-empirical results appear to support the known electrophilic reactivity of the phenylcyanates at the cyanato carbon, C8, the ab initio method is supportive but the substituents surprisingly appear to exert little electronic effect. Because of this unexpected result, the reactivities of the three phenylcyanates were explored by calculating their reactions with a typical nucleophile in an appropriate solvent.

13

J.O. Morley, M. NajtYJoumal of Molecular Structure (Theochem) 401 (I 997) 69-76

Table 2 Atomic charges, energies, and dipole moments of phenylcyanate semi-empirical and ab initio levels Atomic charge

Cl c2 c3 c4 c5 C6 07 C8 N9 NIO XII X12 H2 H3 H4 H5 H6 gc” Gb Hf’

(la) and the 4-nitro (lb) and the 4-amino (lc) derivatives

at the

(lc)

(lb)

(la)

calculated

AM1

PM3

6-31G**

AMI

PM3

6-31G**

AM1

PM3

6-31G**

0.046 -0.131 -0.105 -0.140 -0.101 -0.164 -0.070 -0.025 -0.045

0.045 -0.1 I7 -0.07 I -0.114 -0.067 -0.142 0.045 a.020 -0.085

0.350 -0.162 a.144 -0.151 4.144 -0.182 -0.610 0.584 -0.429

0.156 0.142

0.125 0.108

0.186 0.167

0.093 -0.145 -0.042 a.150 -0.035 -0.181 -0.059 -0.033 -0.018 0.572 -0.353 -0.353 0.173 0.181

0.106 -0.142 0.022 -0.428 0.029 -0.169 XI.003 -0.033 -0.055 1.310 -0.595 a.595 0.137 0.138

0.379 -0.179 -0.091 0.117 -0.093 -0.194 -0.605 0.585 -0.411 0.530 -0.468 -0.467 0.207 0.234

-0.010 -0.085 -0. II7 0.063 -0.173 -0.113 -0.069 -0.024 -0.054 -0.331 0.192 0.192 0.154 0.141

-0.007 -0.071 -0.128 -0.080 a.124 -0.091 -0.012 -0.018 -0.093 0.08 1 0.034 0.034 0.122 0.115

0.305 -0.138 -0.193 0.334 -0.196 -0.152 -0.615 0.581 -0.438 -0.800 0.308 0.308 0.185 0.155

0.140 0.142 0.154 3.312

0.110 0.109 0.129 3.636

0.181 0.170 2.526

0.139 0.140 2.671

0.1 16 0.123 4.822

0.157 0.199 6.476

40.608

46.470

33.295

0.237 0.220 2.296 2.27 Ed

0.141 0.152 4.816

41.234

0.163 0.170 0.200 4.788 3.93 Ed

a Calculated dipole moment (D). b Experimental dipole moment (D) from Ref. [22] ’ Heat of formation (kcal mol-‘). d E is the total molecular energy (in au): (la) = -397.273388:

3.2. Reaction

coordinate

(lb) = -600.739371;

calculations

The energetics of the nucleophilic displacement reaction (Eq. (2)) were explored using the COSMO method [ 181 (implemented in the MOPAC program [ 151 at the AM 1 level [ 13]), which is based on a continuum approach where the solute is embedded in a dielectric continuum of permittivity E. The reactions were carried out between the respective phenylcyanate and the methoxide ion as the representative nucleophile with methanol as the continuum (e = 32.6), and adopting the recommended number of segments per atom on the solvent-accessible surface [ 181. The transition state was approached from both sides of the reaction, with the methoxide ion initially placed perpendicularly at a distance of 10 A from the cyanato-carbon at the starting point for the saddle state calculation, and the displaced phenoxide ion placed perpendicularly at a distance of 10 A from

39.648

38.381

Ed

(lc) = -452.304596.

the same carbon at the end of the reaction. The approximate transition states obtained by this procedure were optimised and characterised in each case by their single imaginary frequency for the reaction coordinate mode (see Section 2). The nucleophilic attack of methoxide ion on each of the phenylcyanates is predicted to proceed in each case via the formation of a stable intermediate (4) which is lower in energy than the reactants (Scheme 2). In terms of the thermodynamics of this reaction, the process is predicted to be exothermic, with the 4-nitro derivative (lb) showing the largest energy difference between the reactants and the intermediate (4b), the 4-amino derivatives (lc) and (4~) the least, with the unsubstituted derivatives (la) and (4a) lying between the two extremes, with values of -9.80, -4.42, and -5.10 kcal mol-‘, respectively, calculated in methanol (Fig. 1, Table 3). As far as the activation energy of this initial reaction is concerned, the

74

J.O. Morley, M. NajVJournal of Molecular Structure (Theochem) 401 (1997) 69-76

R (1)

Scheme 2. Proposed mechanism transition states respectively)

(4)

for the reaction of phenylcyanates

calculations predict transition states which are 17.3 to 18.5 kcal mall’ higher in energy than the reactants (Fig. 1, Table 3). The ring substituents appear, therefore, to exert little effect on the activation energy of the nucleophilic addition reaction to give intermediate (4). These results support the same trends predicted from the positive atomic charge calculated at the cyanatocarbon (C8) using the 6-31G** basis set in the phenylcyanates alone (Table 2), which was found to be largely unaffected by ring substitution (see Section 3.1).

-60 -70 -80

TSl

i 3

E ; s ‘.5 [ y. % 4z

R

R

‘I’S1

-120 -130 ‘u -140

Products

IO 5 1.9 MeO-CN Distance

10 1.7 5 NC-OPh Distance

Reaction Coordinate Displacement Fig. 1. Energetics of the reaction of phenylcyanates (1) with methoxide ion in methanol along the reaction coordinate (distances are given from those found in the intermediate (4)).

‘l-S2 (1) with methoxide

ion in methanol (TSl and TS2 are the initial and final

The initial transition state (TSl) is clearly reactantlike in each case studied, with the NC-OAr bond length stretched from 1.34 A in the phenylcyanate alone to 1.36 A; the MeO-CN distance ranges from 1.91 to 1.96 A, depending on the ring substituent (Table 3). In the intermediates (4), the NC-OAr and MeO-$N bond lengths are comparable at 1.40 to 1.45 A, with the former slightly shorter than the latter in the nitro derivative (lb), although the reverse is found for the other derivatives (la) and (1~). The negative charge on the intermediate (4) resides predominantly at the cyanato nitrogen atom N9 with values ranging from - 0.741 for the nitro derivative (4b) to - 0.772 for the amino derivative (4~). As the methoxide ion is moved closer to the cyanato-carbon of the phenylcyanates to form the products, the energy rises and a second transition state (TS2) is found between the intermediate (4) and the product combination of methylcyanate (3) and the phenoxide ion (Scheme 2, Fig. 1). Thermodynamically, this process is predicted to be strongly exothermic, with the 4-nitro derivative (lb) showing the largest energy difference between the intermediate (4b) and products, the unsubstituted derivative (4a) next in order, and the 4-amino derivative (4~) the smallest, with values of -22.8, -15.7, and -13.8 kcal mall’ respectively (Table 3). The activation energy of this process is much smaller than the addition reaction, though here there are significant differences found between phenylcyanate and its substituted derivatives. The transition state for the conversion of the 4-nitrophenylcyanate intermediate (4b) into methylcyanate (3) and the 4-nitrophenol ion is predicted to be only 3.23 kcal mall’ higher in

15

J.O. Morley, M. Naji/Joumal of Molecular Structure (Theochem) 401 (1997) 69-76

Table 3 Enthalpies

of the reactions

vibrational

frequencies, R’

la

lb

IC

H

NO2

NH2

of phenylcyanates

calculated Coordinate position

(1) with methoxide

ion in methanol,

transition

state geometries,

and reaction

coordinate

using the AMI and COSMO methods HFb

Frequency’

Relative energyd

TS geometry ’ MeO-CN

NC-OAr

=I

R TSI

-101.3 -84.0

to 1.96

1.34 1.35

0 17.3

4a TS2 P R TSI 4b TS2 P R TS1

-106.4 -97.9 -122.1 -105.5 -87.9 -115.3 -112.1 -138.2 -109.0 -90.5

1.41 1.36 1.33 m 1.91 I .45 1.36 1.33 30 1.95

I .43 I .75 0) 1.34 I .36 I .40 1.71 SD 1.34 1.35

-5.10

4c TS2

-I 13.4 -104.6

1.41 1.35

1.44 1.81

P

-127.2

1.33

30

-15.7 0 17.6 -9.80

=2

426.51 0 8.49

564.51 0 3.23

-22.8 0 18.5 -4.42

576.8i

577.1i

47 1.6i 0 8.87

529.21

-13.8

a R and P are the reactants and products respectively; (4) is the intermediate and TS I and TS2 are the transition states on the reactant and product sides of the reaction respectively. b Heat of formation in kcal mall’. ’ Bond lengths in A. d 6E1 and 6E2 are the energy differences calculated from the reactants and intermediate respectively (both in kcal mol-‘). e Wavenumbers in cm-‘.

energy than the intermediate, while the corresponding barrier for the parent (4a) and 4-amino derivative (4~) are much higher at 8.49 and 8.87 kcal mol-’ respectively (Table 3). The second transition state (TS2) is product-like in each case, with NC-OMe bond lengths of around 1.36 A versus 1.33 i in methylcyanate (3). Here the ArO-CN distance ranges from 1.71 A for the TS2 of 4-nitrophenylcyanate (lb) to 1.81 A in the TS2 of the j-amino derivative (lc) (Table 3).

4. Conclusions The rate-controlling step of the nucleophilic substitution reactions of the phenylcyanates (1) is controlled by the addition of methoxide ion to the cyanato group to give a stable intermediate (4); ring substituents do not appear to greatly influence this process. Thermodynamically, however, the substituents do exert an effect and the 4-nitrophenylcyanate intermediate (4b) is favoured over the others. Once the

intermediate (4) is formed, the ring substituents play a role both in the kinetics and thermodynamics of the subsequent displacement of the phenoxide ion. Here the nitro group facilitates this process, both by lowering the energy barrier and increasing the stability of the products. References [l] S. Parai (Ed.), The Chemistry of Cyanates and Their Thio Derivatives, Wiley, New York, 1977. [2] German P. 1079650/1960 (to Bayer). [3] US Patent 40962720978 (to Stauffer Chemicals). [4] E. Grigat, R. Putter, Angew. Chem. Int. Edn., 6 (1967) 206. [5] E. Grigat, R. Putter, Chem. Ber., 97 (1964) 3012, 3022, 3027. [6] D. Martin, S. Rackow, Chem. Ber., 98 (1965) 3662. [7] E. Grigat, R. Putter, Chem. Ber., 97 (1964) 3018. [8] D. Martin, A. Berger, J. Prakt. Chem., 315 (1973) 289. [9] H. Hoyer, Chem. Ber., 94 (1961) 1042. [IO] E.S. Gould, Mechanism and Structure in Organic Chemistry, Halt, Rinehart and Winston, Inc., New York, 1964, p.220227. [I I] J. March, Advanced Organic Chemistry, Wiley, New York, 1985, p.242-250.

76

J.O. Morley, M. Naji/Joumal of Molecular

[ 121 D. Poppinger, L. Radom, J. Am. Chem. Sot., 100 (1978) 3674. [13] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Sot., IO7 (1985) 3902. [14] J.J.P. Stewart, J. Comp. Chem., 10 (1989) 209. (151 MOPAC 93 (J.J.P. Stewart and Fujitsu Limited, Tokyo, Japan: copyright (Fujitsu Limited, 1993) obtained from QCPE, Department of Chemistry, Indiania University, Bloomington, Indiana 47405, USA. [16] See, forexample: W.J. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, Ab Initio Molecular Orbital Theory, John Wiley and Sons, New York, 1986.

Structure (Theochem) 401 (1997) 69-76 [ 171 M.F. Guest, P. Sherwood, GAMES, an ab initio program. The Daresbury Laboratory, Warrington, UK. [ 181 A. Klamt, G. Schuurmann, J. Chem. Sot. Perkin Trans., 2 (1993) 799. [ 191 Cambridge Structural Database, Cambridge Crystallographic Data Centre, University Chemical Laboratory, Lenslield Road, Cambridge, CB2 2EW, UK. [20] L. Kutschabsky. H. Schrauber, Krist. Tech., 8 (1973) 217. [21] J.M.R. Davies, I. Hamerton, J.R. Jones, DC. Povey, J.M. Barton, J. Cryst. Spectrosc., 20 (1990) 285. [22] A. Mansingh, J. Chem. Phys., 5 1 (1969) 2762.