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The reaction pathway for the hydration of ketenimine by water dimer. An ab initio study.
Journal o f Molecular Structure, 93 (1983) 329--332 THEOCHEM
329
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE REACTION PATHWAY FOR THE HYDRATION OF KETENIMINE BY WATER DIMER. AN AB INITIO STUDY. Minh Tho NGUYENI and Anthony F. HEGARTY2 1Department of Chemistry, University of Leuven, B-3030 Heverlee, Belgium. 2Chemistry Department, University College, B e l f i e l d , Dublin 4, Ireland.
ABSTRACT The reaction pathwa~ for the hydration of the ketenimine CHp=C=NH by H20 and (H20)2 has been investigated by ab inJJc~o methods using STO-3G-and 4-31G basis sets, optimized geometries of stationary points being determined by the gradient method. The preferred reaction is with the water dimer which, although i t enters the t r a n s i t i o n state c o r r e c t l y oriented for proton transfer to the B-carbon, does not a c t u a l l y t r a n s f e r t h i s proton u n t i l a f t e r the t r a n s i t i o n state. The reaction is calculated (4-31G) to have an activation b a r r i e r of 22 kcal/mole, most of which arises due to the deformation of ketenimine and water dimer. The reaction with water monomer is f a r less favoured (activation b a r r i e r 67 kca]/ mole using the same basis set). INTRODUCTION Cumulenes of the general type ~C=C=X (X= N,O,S) are highly reactive undergoing rapid nucleophilic or e l e c t r o p h i l i c addition. We have previously reported that the preferred s i t e of protonation of the nitrogen member of t h i s series ( the ketenimine CH2=C=NH) i f the B-carbon rather than the nitrogen atom1. There is also evidence from reactions studied in aqueous solution 2 that the principal reaction pathway is through rate determining proton transfer (from H3O+ or H20 in water) from an electrophile to the B-carbon. We now report an ab ~ o study of the reaction pathway using both water (H20) and water dimer (H20)2 as reactants. Since the results with the l a t t e r reactant predict a reaction with a signif i c a n t l y lower activation energy (and approximate most closely the situation in solution), these are presented f i r s t . DETAILS OF CALCULATION The basis sets of Pople's group were used f o r SCF calculations : STO-3G3 and 4-31G4.The molecular geometries have been optimized by the force method with analytical gradient 5 adapted from the Monstergauss program 6. The t r a n s i t i o n states are localized by the calculation of the second derivatives of the energy with respect to a l l structural parameters of the supermolecule.Charge centroides were obtained using the method of Foster-Boys~
0166-1280/83/$03.00
© 1 9 8 3 Elsevier Science Publishers B.V.
330
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RESULTS AND DISCUSSION The optimal geometry and molecular properties of the ketenimine have p r e v i ously been presented 1'8. The geometric parameters for the t r a n s i t i o n state of the reaction CH2=C=NH + 2H20 are given by 2 in the figure I . The c a l c u l a t i o n s show that the t r a n s i t i o n state is approached with the two molecules of water in the form of a l i n e a r dimer, a struture which has already been shown9 to be most stable among the possible strutures of the dimer. All of the atoms the dimer l i e however in the CCN plane w h i l e , as the oxygen atom, 01 , moves towards the cent r a l carbon atom (Ca), the second water molecule is c o r r e c t l y oriented to f a c i l i t a t e proton t r a n s f e r to the adjacent carbon atom C~ (rather than to the n i trogen of the ketenimine). The CeOl distance at the t r a n s i t i o n state is r e l a t i vely long (1.93 ~) but comparable to that observed f o r the reaction of f u l m i n i c acid (HCNO) with water I0. In analysing the movement of charge centroides of localized o r b i t a l s , we can summarise the electron movement as follows :
.
H
.... C~. 'N ~
"11 (02 )
H
~
o"
(CaO 1 )
T~ (CctCi3) ---,,. o-(CI3H 2) o" (O2H 2) ---,,. ~q (02) (O2) ~ o" ( O2H 1 )
H
o" (O1H 1) --,,- "~ (O 1 ) There are thus f i v e electron pairs implicated in t h i s c y c l i c reorganisation, i n c l u d i n g 6 electrons of
~-type which determine the aromatic character of the
t r a n s i t i o n state 2. In order to describe the evolution of the supermolecule a f t e r t r a n s i t i o n state, we have also calculated the geometry with a f i x e d d i s tance C~O1 ~ 1.92 X ( structure 3 in f i g u r e i ) . We note that at the t r a n s i t i o n I
state only the C~OI bond is formed to any degree; the CBH2 bond does not e x i s t while l i t t l e
or not proton t r a n s f e r has occurred from 01 to 02 . However j u s t
a f t e r the t r a n s i t i o n state (see 3) both protons have "jumped" and the CBH2 and 02H2 bonds are now e s s e n t i a l l y completely formed (while the C~01 bond distance has only decreased by 0.01A).Thus in t h i s addition reaction, the transfer of 2 protons is simultaneous and occurs j u s t at or a f t e r
the t r a n s i t i o n state during
a period when the reaction (as measured by the C~OI distance) has progressed but little
along the reaction coordinate. We have previously shownI0 that STO-3G
c a l c u l a t i o n s of the reaction pathway f o r the related cumulene f u l m i n i c acid,HCNq. closely correspond to the experimental results when j u s t one water molecule is used as reactant. We have therefore recalculated the reaction pathway f o r the reaction of ketenimine with one molecule of water. The p r e d i c t e d t r a n s i t i o n state
332
Table 1 : Calculated Energies for the reaction of ketenimine with one and two molecules of water. Total energies in a.~., r e l a t i v e energies in kcal/mole. STO-3G//STO-3G
4-31G//STO-3G
CH2:C=NH + 2H20
-280.10880
-283.48035
Transition state 2
-280.08470
-283.44521
Activation Barrier
15.1
22.0
CH2:C=NH + H20
-205.14481
-207.57255
Transition state i
-205.05337
-207.46443
Activation Barrier
57.4
66.6
(see 1 in figure 1) is quite d i f f e r e n t from the water dimer reaction. Thus the C~OI bond distance is markedly shorter (1.57 A) while the calculated activation barriers are markedly higher (57.4 kcal/mole in STO-3G and 66.6 kcal/mole in 4-31G with respect to 15.1 in STO-3G and 22.0 kcal/mole in 4-31G, see Table I ) . The deformation of the ketenimine is not a major factor contributing to this high b a r r i e r (15-20% of the b a r r i e r height). Thus the transfer of the proton from oxygen to carbon requires the major part of the energy and in fact the act i v a t i o n b a r r i e r is akin to that found in other 1,3-proton transfer reactions 11. In conclusion, the calculated energy pathway f o r reaction of the ketenimine with water dimer is c l e a r l y favoured over that with one molecule of water and most closely approximates the experimental results found in aqueous solution . ACKNOWLEDGEMENTS : MTN thanks Prof. L.G. Vanquickenborne f o r advice and is indebted to the Belgian Government ( Programmatie van her Wetenschapsbeleid ). The calculations were carried out in University of ZUrich (Switzerland), we thank Prof. G. H. Wagni~re for help. REFERENCES I ~ K . Ha and M.T. Nguyen, J. Mol. S t r u c t . , Theochem, 87 (1982) 355. 2 D.G. McCarthy and A.F. Hegarty, J. Chem. Soc.,Perkin I I (1980) 579. 3 W.J. Hehre, R.F. Stewart and J.A. Pople, J. Chem. Phys., 51 (1969) 2652. 4 R. D i t c h f i e l d , W.J. Hehre and J.A. Pople, J. Chem. Phys., 56 (1972) 2257. 5 H.B. Schlegel, S. Wolfe and F. Bernadi, J. Chem. Phys., 63 (1975) 3622. 6 M.R. Peterson, R.A. P o i r i e r and I.G. Csizmadia, Monstergauss program, Toronto. 7 S.F. Boys, Rev. Mod. Phys., 35 (1963) 457. 8 J. Kaneti and MoT. Nguyen, J. Mol. S t r u c t . , Theochem, 87 (1982) 205. 9 G. Leroy, G. Louterman-Leloup and P. Ruelle, Bull.Soc.Chim.Belg.,85(1976) 205. M.T. Nguyen, M. Sana, G. Leroy, K.j. Dignam and A.F. Hegarty, J. Amer. Chem. Soc., 102 (1980) 573. 11M.T. Nguyen, M. Sana and G. Leroy, Bull.Soc.Chim.Belg., 90 (1981) 681.
329
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE REACTION PATHWAY FOR THE HYDRATION OF KETENIMINE BY WATER DIMER. AN AB INITIO STUDY. Minh Tho NGUYENI and Anthony F. HEGARTY2 1Department of Chemistry, University of Leuven, B-3030 Heverlee, Belgium. 2Chemistry Department, University College, B e l f i e l d , Dublin 4, Ireland.
ABSTRACT The reaction pathwa~ for the hydration of the ketenimine CHp=C=NH by H20 and (H20)2 has been investigated by ab inJJc~o methods using STO-3G-and 4-31G basis sets, optimized geometries of stationary points being determined by the gradient method. The preferred reaction is with the water dimer which, although i t enters the t r a n s i t i o n state c o r r e c t l y oriented for proton transfer to the B-carbon, does not a c t u a l l y t r a n s f e r t h i s proton u n t i l a f t e r the t r a n s i t i o n state. The reaction is calculated (4-31G) to have an activation b a r r i e r of 22 kcal/mole, most of which arises due to the deformation of ketenimine and water dimer. The reaction with water monomer is f a r less favoured (activation b a r r i e r 67 kca]/ mole using the same basis set). INTRODUCTION Cumulenes of the general type ~C=C=X (X= N,O,S) are highly reactive undergoing rapid nucleophilic or e l e c t r o p h i l i c addition. We have previously reported that the preferred s i t e of protonation of the nitrogen member of t h i s series ( the ketenimine CH2=C=NH) i f the B-carbon rather than the nitrogen atom1. There is also evidence from reactions studied in aqueous solution 2 that the principal reaction pathway is through rate determining proton transfer (from H3O+ or H20 in water) from an electrophile to the B-carbon. We now report an ab ~ o study of the reaction pathway using both water (H20) and water dimer (H20)2 as reactants. Since the results with the l a t t e r reactant predict a reaction with a signif i c a n t l y lower activation energy (and approximate most closely the situation in solution), these are presented f i r s t . DETAILS OF CALCULATION The basis sets of Pople's group were used f o r SCF calculations : STO-3G3 and 4-31G4.The molecular geometries have been optimized by the force method with analytical gradient 5 adapted from the Monstergauss program 6. The t r a n s i t i o n states are localized by the calculation of the second derivatives of the energy with respect to a l l structural parameters of the supermolecule.Charge centroides were obtained using the method of Foster-Boys~
0166-1280/83/$03.00
© 1 9 8 3 Elsevier Science Publishers B.V.
330
g
~I
~
I.~ I
--
0
~.~
~
~
40
to
~"
=g, O ~
o( .
¢~
."
At-
~" ~ - . . . .
~
/~o
Z
~
'~o
0
~'-°~5~ ~ ~ g"-
-,-/j~ ~ x:--
t--
O 0 C/. ( ' 4 -'r-
"=: V
OJ
a~
¢j
so
_'1-
°.~
E o 4.-)
tn •~ o ¢o 0~_c: oQ-io
0
I---
~(.0.
~ ~
o f'¢~
T~';T,.'.
.
.,f,...
•1- .
o.
,-
-
,,TJ.o, ~,
X:~--"-mak_o
II Z
II
•
II
0 (.)
v
°"r
V
~
4.-~
T. g ~
oo o
1-
co N
z
~
>~-
4-)4.o
@ ~_.5 ..
?/v -r-veo
S.
---I rid
L1
"-r"
Z
"r V
331
RESULTS AND DISCUSSION The optimal geometry and molecular properties of the ketenimine have p r e v i ously been presented 1'8. The geometric parameters for the t r a n s i t i o n state of the reaction CH2=C=NH + 2H20 are given by 2 in the figure I . The c a l c u l a t i o n s show that the t r a n s i t i o n state is approached with the two molecules of water in the form of a l i n e a r dimer, a struture which has already been shown9 to be most stable among the possible strutures of the dimer. All of the atoms the dimer l i e however in the CCN plane w h i l e , as the oxygen atom, 01 , moves towards the cent r a l carbon atom (Ca), the second water molecule is c o r r e c t l y oriented to f a c i l i t a t e proton t r a n s f e r to the adjacent carbon atom C~ (rather than to the n i trogen of the ketenimine). The CeOl distance at the t r a n s i t i o n state is r e l a t i vely long (1.93 ~) but comparable to that observed f o r the reaction of f u l m i n i c acid (HCNO) with water I0. In analysing the movement of charge centroides of localized o r b i t a l s , we can summarise the electron movement as follows :
.
H
.... C~. 'N ~
"11 (02 )
H
~
o"
(CaO 1 )
T~ (CctCi3) ---,,. o-(CI3H 2) o" (O2H 2) ---,,. ~q (02) (O2) ~ o" ( O2H 1 )
H
o" (O1H 1) --,,- "~ (O 1 ) There are thus f i v e electron pairs implicated in t h i s c y c l i c reorganisation, i n c l u d i n g 6 electrons of
~-type which determine the aromatic character of the
t r a n s i t i o n state 2. In order to describe the evolution of the supermolecule a f t e r t r a n s i t i o n state, we have also calculated the geometry with a f i x e d d i s tance C~O1 ~ 1.92 X ( structure 3 in f i g u r e i ) . We note that at the t r a n s i t i o n I
state only the C~OI bond is formed to any degree; the CBH2 bond does not e x i s t while l i t t l e
or not proton t r a n s f e r has occurred from 01 to 02 . However j u s t
a f t e r the t r a n s i t i o n state (see 3) both protons have "jumped" and the CBH2 and 02H2 bonds are now e s s e n t i a l l y completely formed (while the C~01 bond distance has only decreased by 0.01A).Thus in t h i s addition reaction, the transfer of 2 protons is simultaneous and occurs j u s t at or a f t e r
the t r a n s i t i o n state during
a period when the reaction (as measured by the C~OI distance) has progressed but little
along the reaction coordinate. We have previously shownI0 that STO-3G
c a l c u l a t i o n s of the reaction pathway f o r the related cumulene f u l m i n i c acid,HCNq. closely correspond to the experimental results when j u s t one water molecule is used as reactant. We have therefore recalculated the reaction pathway f o r the reaction of ketenimine with one molecule of water. The p r e d i c t e d t r a n s i t i o n state
332
Table 1 : Calculated Energies for the reaction of ketenimine with one and two molecules of water. Total energies in a.~., r e l a t i v e energies in kcal/mole. STO-3G//STO-3G
4-31G//STO-3G
CH2:C=NH + 2H20
-280.10880
-283.48035
Transition state 2
-280.08470
-283.44521
Activation Barrier
15.1
22.0
CH2:C=NH + H20
-205.14481
-207.57255
Transition state i
-205.05337
-207.46443
Activation Barrier
57.4
66.6
(see 1 in figure 1) is quite d i f f e r e n t from the water dimer reaction. Thus the C~OI bond distance is markedly shorter (1.57 A) while the calculated activation barriers are markedly higher (57.4 kcal/mole in STO-3G and 66.6 kcal/mole in 4-31G with respect to 15.1 in STO-3G and 22.0 kcal/mole in 4-31G, see Table I ) . The deformation of the ketenimine is not a major factor contributing to this high b a r r i e r (15-20% of the b a r r i e r height). Thus the transfer of the proton from oxygen to carbon requires the major part of the energy and in fact the act i v a t i o n b a r r i e r is akin to that found in other 1,3-proton transfer reactions 11. In conclusion, the calculated energy pathway f o r reaction of the ketenimine with water dimer is c l e a r l y favoured over that with one molecule of water and most closely approximates the experimental results found in aqueous solution . ACKNOWLEDGEMENTS : MTN thanks Prof. L.G. Vanquickenborne f o r advice and is indebted to the Belgian Government ( Programmatie van her Wetenschapsbeleid ). The calculations were carried out in University of ZUrich (Switzerland), we thank Prof. G. H. Wagni~re for help. REFERENCES I ~ K . Ha and M.T. Nguyen, J. Mol. S t r u c t . , Theochem, 87 (1982) 355. 2 D.G. McCarthy and A.F. Hegarty, J. Chem. Soc.,Perkin I I (1980) 579. 3 W.J. Hehre, R.F. Stewart and J.A. Pople, J. Chem. Phys., 51 (1969) 2652. 4 R. D i t c h f i e l d , W.J. Hehre and J.A. Pople, J. Chem. Phys., 56 (1972) 2257. 5 H.B. Schlegel, S. Wolfe and F. Bernadi, J. Chem. Phys., 63 (1975) 3622. 6 M.R. Peterson, R.A. P o i r i e r and I.G. Csizmadia, Monstergauss program, Toronto. 7 S.F. Boys, Rev. Mod. Phys., 35 (1963) 457. 8 J. Kaneti and MoT. Nguyen, J. Mol. S t r u c t . , Theochem, 87 (1982) 205. 9 G. Leroy, G. Louterman-Leloup and P. Ruelle, Bull.Soc.Chim.Belg.,85(1976) 205. M.T. Nguyen, M. Sana, G. Leroy, K.j. Dignam and A.F. Hegarty, J. Amer. Chem. Soc., 102 (1980) 573. 11M.T. Nguyen, M. Sana and G. Leroy, Bull.Soc.Chim.Belg., 90 (1981) 681.