Semiempirical and ab initio calculations on geometry and stability of intermediates. Stability of intermediates for nucleophilic reactions of carbonyl compounds in the gas phase and in solution

Journal of Molecular Structure (Theochem), 133 (1985) 263--268 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SEMIEMPIRICAL AND AB INITIO CALCULATIONS ON GEOMETRY AND STABILITY OF INTERMEDIATES. STABILITY OF INTERMEDIATES FOR NUCLEOPHILIC REACTIONS OF CARBONYL COMPOUNDS IN THE GAS PHASE AND IN SOLUTION

KONSTANTIN YA. BURSHTEIN and ALEXANDER N. ISAEV

N. D. Zelinsky Institute of Organic Chemistry, Academy of Sciences of U.S.S.R., Leninsky Prospect 4 7, 117913 Moscow (U.S.S.R.) (Received 7 May 1985)

ABSTRACT Semiempirical CNDO/2, CNDO/BW, MINDO/2, MINDO/3, MNDO and MNDO/H, ab initio STO-3G, 4-310 and 6-31G* methods have been used to calculate the stability and equilibrium geometries of intermediates for nucleophilic reactions of carbonyl compounds in the gas phase and in solution. The solvent effects have been introduced within the supermolecular approach. The limitations of semiempirical methods have been examined. It was found that the MNDO results are in good agreement with the ab initio 4-31G and 6-31G* data. The solvent may change the gas phase data in a remarkable way. INTRODUCTION

A large fraction of chemical reactions and an even larger fraction of biochemical reactions involve the reversible addition of nucleophilic reagent to the electrophilic carbon atom o f the C=O group. One or more proton transfers between the reagents and solvent may accompany the addition and lead to formation of different intermediates [1] 0-

O-

I

I

---C--

--(3--

I

Null ÷ (T ±)

i

(1)

Nu (T-)

Null + ~ C = O OH

OH

I

I

--C--

--C--

I

I

Null ÷ (T +)

Nu (T °)

METHOD

Experimental methods are extensively used in studying these complex reactions, but their mechanism is poorly understood, largely because they 0166-1280/85/$03.50

© 1985 Elsevier Science Publishers B.V.

264

involve the formation of many intermediates. The intermediates may be investigated by the methods of molecular orbital theory. Where computationally feasible, ab initio procedures with extended basis are preferred for the study of the reaction mechanism. In the case of complex chemical and biochemical systems semiempirical procedures provide a useful and economical alternative. In the present paper semiempirical and ab initio results are compared to test the methods and to determine some possible limitations of the applicability of the semiempirical methods. The CNDO/2 [2], CNDO/BW (parametrization 1) [3], MINDO/2 [4], MINDO/2' [5], MINDO/3 [6], MNDO [7] and ab initio STO-3G, 4-31G, 6-31G and 6-31G* [8--10] methods were tested. The geometry was optimized by the semiempirical and ab initio 4-31G methods. The MNDO and 4-31G optimized geometries were then used for single-point energy calculations by the STO-3G, 6-31G and 6-31G* methods. The solvent effects were introduced within the supermolecular approach. Two, four and six water molecules were added in the reaction system. The MNDO/H method [11] was used to calculate the geometry and stability of the complexes with water molecules. The calculations were performed for the simplest model reaction NH3 + H2CO ~ T ± , T +, T-, T O RESULTS

AND

(2)

DISCUSSION

All the semiempirical and ab initio methods give the local minima for the intermediates T °, T-, T* in the gas phase. The results of our calculations are listed in Tables 1 and 2. They show good agreement for the CNDO/BW, MINDO/2, MINDO/3, MNDO and ab initio 4-31G methods. The CNDO/2 method overestimates the stability of the intermediates. To investigate the stability of the zwitterion intermediate T ± we calculated the path for the following reaction NH3 + H2CO -~ H3N+-C--O - (T ±) TABLE

(3)

1

Calculated energies of intermediates (kcal mo1-1) Method

CNDO/2 CNDO/BW MINDO/2 MINDO/2' MINDO/3 MNDO Ab initio 4-31G

T Oa

--589 --270 --278 --277 --293 --284 --308

T-b

--262 --77 --71 --63 --81 --56 --50

aTotal energy of isolated N H 3 and H 2 C O is assumed to be equal to zero. bTotal energy of isolated N H ~ and H 2 C O is assumed to be equal to zero. CTotal energy of isolated N H s and H 2 C O H + is assumed to be equal to zero.

T +c

--274 --76 --65 --68 --67 --61 --71

265 TABLE 2 Geometry and electronic structure of intermediates Intermediate

Bond lengths (A) C--O C--N

Angle O C N (grad)

Atomic charges C 0

N

1.39 1.32 1.38

1.47 1.54 1.70

106 115 103

0.239 0.353 0.396

--0.751 --0.899 --0.222

--0.858 --0.832 --0.903

T O • 3H20 TT-. 3H20 T* T ~ . 4H20

1.39 1.39 1.29 1.33 1.37 1.37

1.47 1.47 1.53 1.53 1.55 1.54

106 106 110 120 103 103

0.265 0.271 0.408 0.350 0.295 0.292

--0.340 --0.368 --0.750 --0.724 --0.325 --0.359

--0.323 --0.329 --0.423 --0.388 --0.015 --0.162

MINDO/3 TO TT~

1.35 1.26 1.33

1.42 1.46 1.48

109 115 105

0.537 0.725 0.457

---0.484 --0.815 --0.431

--0.202 --0.299 --0.182

CNDO/2 TO TT~

1.37 1.34 1.36

1.41 1.43 1.44

109 113 107

0.271 0.321 0.242

--0.248 --0.664 ---0.222

--0.213 --0.240 --0.027

Ab initio 4-31G To TT* MNDO

TO

30.

20.

~'.'5

t.'7

I.~

2:'

~/'Tc,u, A°

Fig. 1. Potential energy profiles for the reaction (3): 1, STO-3G; 2, 4-31G; 3, 6-31G; 4, 6-31G*. Figures 1 and 2 represent plots of complete energy changes during the r e a c t i o n as a f u n c t i o n o f r e a c t i o n c o o r d i n a t e v a l u e R N ...c. I n s p e c t i o n o f t h e s e d a t a r e v e a l s t h a t t h e a b i n i t i o m e t h o d s w i t h d i f f e r e n t b a s e s sets p r e d i c t

266

~, z~¢,;,,d

2D

z./~. 2"/

-20 ¸

Fig. 2. Potential energy profiles for the reaction (3): 1, MNDO; 2, MINDO/2; 3, MINDO/2' ; 4, MINDO/3; 5, CNDO/BW; 6, CNDO/2. the repulsion of the reagents. There is no local minimum for the zwitterion intermediate T ±. This result shows that the zwitterion intermediate T ± cant form in the gas phase. The MNDO method gives similar results. The MNDO repulsion energies are equal to the ab initio 4-31G values. At the MINDO/2 and MINDO/3 levels the formation of T ± is not energetically profitable, b u t local minima were found in the potential curves. This result is in contradiction with the ab initio and MNDO data and it can be concluded that MINDO/2 and MINDO/3 slightly underestimate the repulsion of the reagents. The CNDO/2 and CNDO/BW methods grossly overestimate the stability o f the zwitterion intermediate T ±. The calculated bond lengths and angles are presented in Table 3. These data indicate that the MNDO method gives the best agreement with the ab initio 4-31G values. The supermolecular approach was used to introduce the solvent effects. All calculations were performed using the MNDO method. The modified function for core--core repulsion was used for hydrogen bonds. We used the modified MNDO method (referred to as MNDO/H) as it gives good agreement with ab initio calculations for the potential curves in the gas phase and good agreement with experiment for solvation energies of small molecules and ions. Two, four and six water molecules were added in the reaction system. Their positions and the geometry of the reagents were optimized simultaneously. The hydration sites were located near the --C--O- and --NH3 groups.

267 TABLE 3 Geometry HsN÷ -* H2C--O- as a function of reaction coordinate value R N . . . C

R C N (A)

R C O (A)

O C H (grad)

O C N (grad)

Dipole m o m e n t (Debye)

A b initio4-31G 1.5 1.7 1.9 2.1

1.32 1.28 1.24 1.22

117 119 121 122

110 110 109 109

8.6 7.5 6.4 5.5

MNDO 1.5 1.7

1.27 1.25

116 119

108 106

7.1 5.9

1.9 2.1

1.24 1.23

121 122

102 100

4.7 3.9

MINDO/3 1.5 1.7 1.9 2.1

1.24 1.22 1.20 1.19

121 123 124 126

110 109 108 105

6.5 5.7 4.7 3.9

CNDO/2 1.5 1.7 1.9 2.1

1.32 1.29 1.27 1.26

117 120 121 122

109 108 107 105

8.3 7.0 5.6 4.5

20-

t6-

2

t

42-

Fig. 3. Potential energy profiles: 1, reaction (3); 2, reaction (4); 3, reaction (5); 4, reaction (6).

268 H2CO" H 2 0 + NH3 • H 2 0 -~ T ± " 2 H 2 0

(4)

H2CO" 2 H 2 0 + NH3 " 2 H 2 0 -~ T ± : 4 H 2 0

(5)

H2CO" 3 H 2 0 + NH3 • 3 H 2 0 -~ T ± • 6 H 2 0

(6)

Figure 3 represents plots o f c o m p l e t e e n e r g y changes d u r i n g t h e r e a c t i o n as a f u n c t i o n o f r e a c t i o n c o o r d i n a t e value R N . . . c " As can be seen f r o m Fig. 3, t h e solvent decreases t h e repulsion o f the reagents and local m i n i m a appear in p o t e n t i a l curves f o r t h e r e a c t i o n s (5) and (6). This result shows t h a t the solvent changes the gas phase d a t a in a r e m a r k a b l e w a y and the z w i t t e r i o n i n t e r m e d i a t e T ± m a y exist in solution. REFERENCES 1 W. P. Jencks, Chem. Rev., 72 (1972) 705. 2 J. A. Pople and G. A. Segal, J. Chem. Phys., 44 (1966) 3289. 3 R. J. Boyd and M. A. Whitehead, J. Chem. Soc., Dalton Trans., (1972) 72. 4 M. J. S. Dewar and E. Haselbach, J. Am. Chem. Soc., 92 (1970) 590, 3855. 5 M. J. S. Dewar and D. Lo, J. Am. Chem. Soc., 94 (1972) 5296. 6 R. S. Bingham, M. J. S. Dewar and D. Lo, J. Am. Chem. Soc., 97 (1975) 1285. 7 M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 99 (1977) 4899. 8 W. J. Hehre, R. F. Stewart and J. A. Pople, J. Chem. Phys., 51 (1969) 2657. 9 R. Ditchfiald, W. J. Hehre and J. A. Pople, J. Chem, Phys., 54 (1971) 724. 10 P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 28 (1973) 213. 11 K. Ya. Burshtein and A. N. Isaev, Theor. Chim. Acta, 64 (1984) 397.