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A Gaussian-2 molecular orbital study of the unimolecular reactions of acetyl cyanide
23 May 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 270 (1997) 363-368
A Gaussian-2 molecular orbital study of the unimolecular reactions of acetyl cyanide Suk Ping So Chemistry Department, The Chinese Universityof Hong Kong, Shatin, N.T., Hong Kong
Received 21 November1996;in final form 20 March 1997
Abstract
Some possible unimolecular reactions of CHaC(O)CN in its ground electronic state have been studied with the G2 molecular orbital method. Among the reactions considered, only its decomposition to CO and CH3CN is exothermic (A H = -7.66 kcat mol-I) and its isomerization to CH 3C(O)NChas an activation barrier (40.77 kcal mol-I) accessible at low temperature. CH3C(O)CN is predicted to be rather stable at room temperature in line with experiment since the former reaction channel and the second step of the latter channel by which CH3C(O)NCdecomposes to CO and CH3NCboth have very high barriers, viz. 109.47 and 100.31 kcal mol-1, respectively.
1. Introduction Formyl cyanide HC(O)CN is a molecule of considerable chemical interest. It is the simplest prototype of the acyl cyanides formed from an aldehyde and a cyano group. In addition, it has various potential uses as a formylating agent for nucleophilic species in organic syntheses. Although there were several early attempts to synthesize formyl cyanide, evidence for its existence was not reported until 1986 [1]. This is partly attributed to its thermal instability which has been studied by different experiments [1-4]. Theoretically, ab initio molecular orbital studies on the various possible unimolecular reaction paths (analogous to reactions la, lb and 3 - 6 shown below) of HC(O)CN have been reported recently [5,6]. Results suggest that HC(O)CN is rather stable at room temperature but can decompose into HCN and CO at high temperature. Acetyl cyanide (or pyruvonitrile) CH3C(O)CN is the second member of the acyl cyanide family. Ben-
nett et al. [7] investigated its pyrolysis at 470°C in a flow system. They found that the reaction proceeded competitively via channels 1 and 2: CH3C(O)CN ~ CO + CH3CN,
(la)
CH3C(O)CN ~ CO + CH3NC,
(lb)
CH3C(O)CN ~ CH 2 = C O + HCN.
(2)
Recently, Okada and Saito [8] studied the thermal unimolecular reaction of CH3C(O)CN diluted in Ar behind reflected shock waves over the temperature range of 1014-1300 K. It should be noted that both groups of researchers did not differentiate reaction la from reaction lb. Contrary to the result of Bennett et al. [7], methyl cyanide (or acetonitrile)CH3CN, one of the products of reaction 1, was not found by Okada and Saito [8] in their shock wave study of the unimolecular reaction of acetyl cyanide. They also performed an ab
0009-2614/97/$17.00 Copyright© 1997Elsevier Science B.V. All rights reserved. PH S0009-261 4(97)00367-9
364
S.P. So/Chemical Physics Letters 270 (1997) 363-368
initio molecular orbital calculation on the reaction via paths 1, 2 and 3. CH3C(O)CN ~ CH3C(O)NC.
(3)
The barriers (activation energies) to the reactions 1, 2 and 3 were computed to be 114.1, 78.4 and 58.5 kcal mol -~ at the H F / 3 - 2 1 G + Z P E level. This indicates that unimolecular decompositions of acetyl cyanide are energetically less favourable, and its isomefization to acetyl isocyanide CH3C(O)NC is dominant. The above theoretical study of Okada and Saito [8] on CH3C(O)CN was done only at a rather low level of theory (HF/3-21G) and only three reaction paths were considered. In addition, results [4,5] for HC(O)CN have shown that the energy barriers and geometries, particularly for the transition state structures, differ substantially with the level of theory employed. Accordingly, it is thought desirable to investigate, at a higher level of theory, namely the Gaussian-2 theory, the unimolecular reaction of CH3C(O)CN for the following reaction paths in addition to those mentioned above. CH3C(O)NC ~ CO + CH3NC, CH3C(O)CN ~ CH3OCCN , CH3C(O)CN ~ CH3COCN.
(4) (5) (6)
2. Calculations
The equilibrium and the transition state structures involved in the reactions studied were optimized by the energy gradient method at the HF and the MP2(FU) levels (MP2(FU) denotes MP2 theory with full electron correlation, i.e. including inner shell electrons), using the GAUSSIAN 94 programs [9] implemented on our IBM RS/6000 (Models 320H and 390) and SGI R8000 workstations. The energies of the above optimized stationary point structures were computed at the Gaussian-2 (G2) theory [10] which is the improved Gaussian-I (GI) theory [11,12]. Computationally, the G2 procedure involves the following steps: (a) Geometry optimization at the HF/6-31G* and the MP2(FU)/6-31G* levels;
(b) Calculation of the H F / 6 - 3 1 G * / / H F / 6 31G* vibrational frequencies; (c) Single-point calculation, for the MP2(FU)/631G* geometry, of frozen-core MP4SDTQ and QCISD(T) energies with the 6-311G** basis, MP4SDTQ energies with the 6-311 + G * * and 6311G(2df, p) bases, and MP2 energies with the 6-311 + G(3df,2p) basis. The G2 energy is defined [10] as the G1 energy plus a correction term (A) which eliminates a number of deficiencies present in the G1 theory. The G1 energy [11,12] on the other hand, is just the frozencore M P 4 S D T Q / 6 - 3 1 1 G * * / / M P 2 ( F U ) / 6 - 3 1 G * energy with various corrections. These include a correction ( A E ( + ) ) for the diffuse functions, a correction (AE(2df)) for the higher polarization functions on non-hydrogen atoms, a correction (AE(QCI)) for the correlation effect beyond the fourth-order perturbation theory, a 'higher level correction' (A E(HLC)) for the remaining basis set deficiencies (G1 and G2 have different AE(HLC) values), and the zero-point vibrational energy (ZPE); or, equivalently, the GI energy is the approximate energy of a full calculation at the frozen-core QCISD(T)/6-311 + G(Zdf,p)//MP2(FU)/6-31G* level plus the A E(HLC) and the ZPE corrections. The connection between each transition state structure and the reactant and the product(s) was established by intrinsic reaction coordinate (IRC) calculations based on the reaction path following algorithm of Gonzalez and Schlegel [ 13,14] as coded in Gaussian 94, or by optimization starting from a transition state structure with one or two of its geometrical parameters distorted.
3. Results and discussion The various stationary point structures studied are depicted in Figs. 1-3 together with their optimized geometrical parameters. These structures have been confirmed to be either equilibrium structures (1, 2, 3 and 4) or transition state structures (TSIa, TS3, TS4, TS5 and TS6) by computing their HF/6-31G* vibrational frequencies. Okada and Saito [8] have calculated the structures of CH3C(O)CN 2 and the transition states TSla, TS2 and TS3 for the reactions l a, 2 and 3, respec-
S.P. So/Chemical Physics Letters 270 (1997) 363-368 He
Hc
i.oeo\
(1~o>,, ~ /" .01~
H
1.0~0 H 10~1 <,=,'x, 1~0 1.079 ~¢6 (,~,.o>~),.;.,,'.~. 1.083
',N1
1•
125.6 (125.2) f ' - - " . / 1.503 ~ll, 4 '.,_ ,.." (1.199) " O--~Ca
,.,,3
lu.J L/
k...\ ,.49o
(149~
(1.211) 120.3~(1.425) 1.1~ (1~0.2) M . , ~ IZIJ N
(I.12~) 119-%\ (I.,tg0) (119.5) ~\ 178.3 (~C (i ~0.2)~ , . ~ I.I ~d. (1,1tl2)
I~.~(~\
(179.4) \ 17&9 \
L CaCbH¢=109.2(109.l)109.4 / CtCbH=I09.8(I09.7)I09.6 I
OCaCbH---~:120.9(+121.0):t120,8
2 (Cs)
1 (Cs) /
1166
c
N L CiCbHc= 109.2( 109. I ) / C,CbHffil09.9(109.9) L OC,Cbl I---~:120.9(_+ 121.0)
H
LI~
(1.19~)
i.trm (1:'~9) i06,5 (l.09a) / C bq' ' ~ " *
f
.
1.42
(114.6)
1,341
o,
o.~71o
1.276~.. \ 1.471 (13041 ~(1.41~)
'°',~k ~'~ (168.O 1,2.3 (106.9) N
3 (C s)
,~c/
(i.4u)
11.09"/) .l~t3
o.~97>
i~ f..-; (,~_~/ <1,',,%'
r,6.4 /'~
075.0) I, " v
(LI~X / OCbH,=109.4(108.6) L OC~H=107.4( 106,7 ) / C ~ O C b l l ~ 120.9(_+ 121.0)
(104.9> 1 1 . 4 8 1 1 " H d
"N/~('1~11~)
L CtCbttc=i 15.4(116.0) / CaCbHd=108.3(106.8)
365
its triple-bond character (1.178 ,~, in CH3CN) during the complete course of the reactions considered. The dissociation reactions of CH3C(O)CN and CH3C(O)NC studied in this work are represented by reactions la, lb, 2 and 4. In contrast to the case of HC(O)CN, attempts to locate the transition state for the reaction lb of CH3C(O)CN have not been successful in this work. The optimized transition state structures T S l a , TS2 and TS4 for the corresponding reactions l a, 2 and 4 are depicted in Fig. 2. As anticipated, the 3-21G geometrical parameters of T S l a and TS2 obtained by Okada and Saito [8] deviate significantly from the values predicted here. Fig. 2 shows that both T S l a and TS4 contain a loose three-membered ring each, while TS2 a loose fourmembered ring comprised of CCCH. The transition state structures T S l a and TS2 are of C~ symmetry, but TS4 becomes C 1 with the C atom of the NC
Z_ C,~CbH~=106.1(106.7) / 0C,CBH¢=+8.7(0.0) L OCaCbt Id =+ 133.7(+ 124.0 )
/ OCaCbH~=-] 13,0(-t23.9) 4 (C1)
I •I09
Ha
2.253
°---rc.,..... ::L. Ig~,'--".9oo,
/~.~ ':~--
Fig. 1. Optimized HF/6-31G * and MP2(FU)/6-31G * (in parentheses) equilibrium structures of isomeric C 3NOH 3. Bond lengths are in angstroms and bond angles in degrees. HF/3-21G values for 2 from Refi [8] are in italics.
'1~'/
tively, at the H F / 3 - 2 1 G level of theory. These are included in Figs. 1-3 for comparison purpose. It is found that their 3-21G geometry of 2 agrees quite well with the present 6-31G * value, indicating that this geometry is rather insensitive to the size of the basis set. However, the agreement is much poorer for the transition state structures T S l a , TS2 and TS3. It should be noted, as pointed out above, that Okada and Saito [8] did not differentiate reaction la from reaction lb, and so did not differentiate the corresponding transition states T S l a and TSIb. The length of the CN bond of the structures shown in Figs. 1-3 is seen to lie between 1.181 and 1.193 ,~ at the MP2(FU)/6-31G* level except those of the transition states TS2, TS3 and TS4, which are o somewhat longer, viz. 1.198, 1.205 and 1.211 A, respectively. This insignificant variation of the CN bond distance between the structures studied, as also noted in the case of HC(O)CN [6], indicates the difficulty of breaking the CN bond, which preserves
L CaCbH=104.3(113.9)117.6
".,~2 .." " ~ " 1 ~
2.692 ; ,." 2.523 19.419> '. , / (2.256> 2.5.tl (',co • 2,412 153.7 ~ , (143.8) 1.150 143.3 ( . 92) N 1335
(1.0~1) 1,066
/" C,CBH¢=81.7(85,8182.9 L CCaCbH=:I:62.1X+67.0)_+62.7
TSla (Cs) L109 0.148)
1,421 (1A24)
1.103 (1,142)
I.II4 1.432 O----~C,--~-----C~
109.6
I-I 1,082 ~ , (I ,093) f g--
;
(111.3)k--~-;,~.s 1o1~,~ i (,~2 (2.795) :
.
1.083 H
~'"
(2.4101 ;
:<115,>., i1. .H e
(LI9g) 1.155
L CjCbHc=100.0(101,0)96.2 L C,C~H=109.5(109.0)110.7 L CCaCbl t ----L-_117.6(+ 118.7)+_117.1
TS2 (Cs)
Hc I" 1I'668 I (
(io~.o> '.~9.s ~7 : .c,,c::~ ....
,.~
2.761 ; .-*° 9.1.3 ~ . +- "" I.,56 (71,1) (~.C'" (1,769) 139.0N~52 I,&T7
2.365
o ,,-77~ ~. .. . . .(21~,> 111.2 ; ....... . , ;
. " 2.494 o°" ('2.323) °* ' / 122.5 N~k)(118.2)
Hd
I.t'~ (1.076)
1,166 "% (1.211> C
/ ' CaCbHc=78.9(82.6) / - CaCbtIn=105.3( 125.4) ~-- CaCbHo=100.0(93.8)
i L L L
OCaO_~t-lc=+165.7(+ 142.9) OCaCbttd=+46.8(+38.41 OCaCbH¢=-75.8(-100.21 CNCbCa=-97.6(-96.0)
TS4 (CI)
Fig. 2. Optimized HF/6-31G * and MP2(FU)/6-31G * (in parentheses) transition state structures for the decomposition reactions of CH3C(O)CN and CH3C(O)NC. Bond lengths are in angstroms and bond angles in degrees. HF/3-21G values for T S l a from Ref. [8] are in italics,
366
S.P. So / Chemical Physics Letters 270 (1997) 3 6 3 - 3 6 8 He I.[~1
1.~2
(i.091)~ 132.0
1.152
Cb.~.,. ~ Ha
(133.7) / ,
o.lsu
09 (l'~i) HQ
i~s6 / 2 " ~ ,
I.IJ7
lzs2
ll.q~j
I 092
I" CaC1~1~=108.4(109.0)109.2 / CaCbHd=108.9( 108.3)108.9
/ CaC~rI¢=I09.4(108.9)I08.9
( '~.':" I ,.','~o I
,.~o
/ / L /
1.925
(1.871) ,fi" ('/0.6) ,(1.886) 1.954 ** 6
67~
2.078
•
OCaCbHd=+120.5(+ 121.0)+120.7 OCaCbHc=-121.0(-121.4)-121.3 NCaCbI-Ie=+155.6(+ 157.8) CC,CbHc=- 165.9(- 163.0)
(i.2os)
t.tzz
T S 3 (CI)
He 1.07$~ Ho 1.1")69 (1.094) " ~ ~ (I.(~l) ,' ',""~DNn 1.0"79 / ",, u.097)
He I.~5~ ~ O.0%) Cb(,~
I ~
1.g43 : ". 85, (I.724)" 696". (1.795)
2.g2(111520~, / i1'47~9 C~1;324) /Z:-~J (. 9)
. (65.3)C', O--~C,
\
(I.313) 1.506 1.251 (I13.1) 113.5{~llI (1.459) ~- CaCbH =120.5(125 5"~
,'c,cb.~=9o0(s837"
/-- CaCbHc=101-2(103.4) / _ OC.CbH¢=+31.0(+31.2)
|'~
1.1~6 O.095) H
1.415 '~~. (I.452)
172,4
(55.6) ," ,"(I.704) 1.613
~'/
157.5
V ('~'5, <,,:,,;~>i ~`'5'~> 1.138~
0.184)~
N
/ OCaCI~Id=+ 145.9(+ 141.4) / OCaCt, H,~=-97.8(-103.1) /-- CCaOCb=+I 12.3(+112.7) L NCC~,O=+176.8(+ 164.5)
L CaCbHc=I 11.2(I I 1.4) . i CaCbtl=108.4(108.3) / OCaCbIl--!--122.1 (+ 122.4)
T S 5 (CI) Fig. 3. O p t i m i z e d H F / 6 - 3 1 G
T S 6 (Cs) * and MP2(FU)/6-31G
* (in paren-
t h e s e s ) transition state s t r u c t u r e s f o r the i s o m e r i z a t i o n r e a c t i o n s o f CH3C(O)CN.
B o n d l e n g t h s are in a n g s t r o m s and b o n d a n g l e s in
degrees. HF/3-21G
v a l u e s f o r T S 3 f r o m R e f . [8] are in italics.
bond sticking out of the plane of the non-hydrogen skeleton (referred to as molecular plane hereafter). Acetyl cyanide has three isomerization pathways. In reaction 3, the migration of the acetyl group from the C atom to the N atom of the CN group results, via the transition state TS3, in the isomer CH3C(O)NC which may decompose into CO and CH3NC, as in the case of HC(O)CN [5,6]. The 1,2-CH 3 and the 1,2-CN shifts from the C atom to the O atom lead, via the transition states TS5 and TS6, to the products CH3OCCN (reaction 5) and CH3COCN (reaction 6), respectively. The transition state TS3 may be taken from Fig. 3 as arising from the migration of the N atom and the twisting of the CN bond out of the molecular plane. Its MP2/631G* CaC and CaN bond distances are 1.871 and 1.886 A, respectively. Further enhancement of the CN interaction and the breaking of the CaC bond lead to the product CH3C(O)NC. When isomerizing to CH3OCCN , CH3C(O)C N goes through the transition state TS5 which involves not only the migration but also the rotation of the CH 3 group and the bulging of the CN bond out of the molecular plane. The isomerization of CH3C(O)CN to CH3COCN , on the other hand, results from an in-plane 1,2-CN shift. The three transition state structures described above involve a loose three-membered ring each (Fig. 3).
Table I E n e r g y c o m p o n e n t s a n d G 2 total e n e r g i e s Species
E(MP4SDTQ/6-311G*
*) a
AE(+)
b
A E(2df) b
AE(QCI)
t,
A b
AE(HLC)
b
Z P E b.c
E(G2) a -245.688337
1
-245.531872
-9.335
- 128.365
+7.913
- 14.632
-65.000
+52.954
2
- 245.509055
- 8.978
- 128.532
+ 6.240
- 15.385
- 65.000
+ 52.440
- 245.668270
3
-245.429668
- 10.548
- 128.178
+4.492
- 14.877
-65.000
+52.474
-245.591305 -245.568291
4
-245.403017
- 10.051
- 130.086
+3.144
- 14.000
-65.000
+50.721
TSIa
-245.349146
- 11.164
- 127.005
+6.079
- 14.178
-65.000
+46.536
-245.513878
TS2
-245.423710
- 12.289
- 128.149
+ 10.736
- 14.849
-65.000
+45.764
-245.587497 -245.623369
TS3
-245.464355
- 10.191
- 126.761
+7.761
- 15.181
-65.000
+50.358
TS4
-245.346625
- 10.895
- 125.384
+8.734
- 15.069
-65.000
+45.815
-245.508424
TS5
- 245.349202
- 11.279
- 129.539
+4.999
- 15.269
- 65.000
+49.582
-245.515708
TS6
-245.364547
- 10.501
- 131.165
+7.061
- 14.915
-65.000
+49.415
CO
- 113.098623
-3.720
-54.202
+5.060
-5.974
-25.000
+4.962
CH3CN
- 132.448614
-3.769
-68.091
+3.331
-9.564
-40.000
+43.666
- 132.523041
CH3NC
- 132.409025
-4.258
-67.731
+2.336
- 10.425
-40.000
+43.505
- 132.485598
-5.151
-80.998
+4.957
-9.835
-40.000
+30.542
- 152.369110
- 3.008
- 46.991
+ 3.008
- 5.124
- 25.000
+ 16.062
- 93.284894
CH2 =COHCN
152.268625 - 93.223841
a In hartrees. b In m i l l i h a r t r e e s . c HF/6-31G
* f r e q u e n c i e s ( e x c l u d i n g i m a g i n a r y o n e s ) s c a l e d b y 0.893 are u s e d to calculate Z P E [12].
-245.529652 - 113.177497
S.P. So / Chemical Physics Letters 270 (1997) 363-368
The energy components and the G2 total energies of the various species studied are listed in Table 1, and the potential energy surface for the unimolecular reactions of CH3C(O)CN is depicted in Fig. 4. It is seen that, at the G2 level of theory, CH3C(O)NC, CH3OCCN, and CH3COCN lie 12.69, 60.89 and 75.33 kcal m o l - ~ higher in energy than CH 3C(O)CN, respectively. This order of relative stability has also been predicted for the case of HC(O)CN [5]. As shown in Fig. 4, among all the decomposition and isomerization reactions considered, the isomerization of CH3C(O)CN to CH3C(O)NC (reaction 3) has the lowest barrier, as in the case of HC(O)CN [5], and is the only reaction feasible at low temperature because of its low barrier of 40.77 kcal mol-1 The other two isomerization reactions 5 and 6 have much larger barriers (99.58 and 108.33 kcal mol-1 respectively) and are thus likely to occur only at very high temperature. That is, with these activation energies, reactions 3, 5 and 6 would have some conveniently measurable specific rate k of, say, 1.5 × 10 .2 s- i when carried out at 600, 1470 and 1600 K, if the pre-exponential factor of Arrhenius equation is set
Potential Energy (kcal/mol)
120.0
~
TS4
TSla
100.0
lL290
TS5 l(f,) 47
r s 6 10g.33 99.58
80.0
H~COCN
60.0
4 75.33
60.~9
TS3 40.0 HCN + 2:0-0
2154 12.39
O.O
co. (
1 oo "
-7.66
Reaction Coordinate
Fig. 4. Semi-quantitative potential energy surface for the unimolecular reactions of CH3C(O)CNcalculated at the G2 level of theory.
367
equal to a median value of 1013 S-t usually found for unimolecular reactions [15,16]. The decomposition of CH3C(O)CN to CO and CH3CN (reaction la) is exothermic by 7.66 kcal moi -1 at the G2 level, yet it is kinetically unfavoured owing to its large barrier of 109.47 kcal mol - l . This is similar to the case of HC(O)CN [6] and is in line with the experimental observation [8] that no acetonitrile CH3CN, one of the products of reaction la, has been detected in the thermal unimolecular reaction of CH3C(O)CN diluted in Ar studied behind reflected shock waves over the termperature range of 1014-1300 K. The decomposition reaction 2 of CH3C(O)CN to HCN and CH 2 = C O is predicted in this work to have an intermediate activation energy of 63.3 kcal moland hence to be feasible at moderately high temperature. The decomposition channel of CH3C(O)CN to CH3NC and CO via the isomeric intermediate CH3C(O)NC which lies 12.59 kcal mol-• higher in energy consists of two successive steps, i.e. reactions 3 and 4 (Fig. 4). Their respective activation energies are 40.77 and 100.31 kcal mol -~, each of which is lower than those of reactions la (109.47 kcal mol- ~). The barrier for CH3C(O)NC to revert back to CH3C(O)CN is only 28.18 kcal tool -t which is significantly lower than that (100.31 kcal tool -l ) for CH3C(O)NC to decompose to CH3NC and CO. This t w o - step reaction channel may be viewed as having a pseudo barrier of about 112.90 kcal mol -~ (Fig. 4) and, only at high temperatures, can be regarded as having the second step, i.e. reaction 4, become the rate-determining step with a barrier of 100.31 kcal m o l - t . Similar results have been predicted for HC(O)CN [5]. From their IR emission data, Okada and Saito [8] obtained a A H value of 5 kcal mol -I for the isomerization (reaction 3) of CH3C(O)CN to CH3C(O)NC, which is in excellent agreement with their H F / 3 - 2 1 G energy difference of 5.2 kcal molbetween CH3C(O)CN and CH3C(O)NC. However, the corresponding value calculated in this work at the G2 level is more than twice as large, viz. 12.59 kcal mol -~. This thus casts some doubt on the reported IR emission data because G2 values are believed to be more reliable than H F / 3 - 2 1 G values in view of the small average absolute error (around
368
S.P. So / Chemical Physics Letters 270 (1997) 363-368
1.2 kcal mol - l ) of G2 energy differences and the low level of theory and limited basis set of the HF/3-21G method [17,18].
Acknowledgements The author thanks the Hong Kong Research Grant Council for an Earmarked grant (RGC REF. CUHK 72/92E) and the Chinese University Research Committee for a Direct Grant to support fully and partially the acquisition of the RS/6000 Model 390 and the SGI R8000 workstations, respectively. The assistance of Mr. Philip Ling of the Chinese University Computer Services Centre in implementing the Gaussian 94 package on the workstations and in taking care of system problems is appreciated.
References [1] R.H. Judge, D.C. Moule, A. Biernacki, M. Benkel, J.M. Ross, J. Rustenburg, J. Mol. Spectros. 116 (1986) 364. [2] D.J. Clouthier, D.C. Moule, J. Am. Chem. Soc. 109 (1987) 6259. [3] M. Bogey, J.L. Destombes, Y. Vallee, J.L. Ripoll, Chem. Phys. Lett. 146 (1988) 227. [4] W. Lewis-Bevan, R.D. Gaston, J. Tyrrell, W.D. Stork, G.L. Salmon, J. Am. Chem. Soc. 114 (1992) 1933.
[5] W.H. Fang, R.Z. Liu, X.Z. You, Chem. Phys. Lett. 226 (1994) 453. [6] N.Y. Chang, C.H. Yu, Chem. Phys. Lett. 242 (1995) 232. [7] R. Bennett, E. Jones and P.D. Ritchie, J. Chem. Soc. (1956) 2628. [8] K. Okada, K. Saito, J. Phys. Chem. 99 (1995) 13168. [9] M.J. Frisch, G.W. Trucks, H.B, Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AI-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. DeFrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez and J.A. Pople, Gaussian 94, Gaussian, Inc., Pittsburgh PA, 1995. [10] L.A. Curtiss, K. Raghavachari, G.W. Trucks, J.A. Pople, J. Chem. Phys. 94 (1991) 7221. [11] J.A. Pople, M. Head-Gordon, D.J. Fox, K. Raghavachari, L.A. Curtiss, J. Chem. Phys. 90 (1989) 5622. [12] L.A. Curtiss, C. Jones, G.W. Trucks, K. Raghavachari, J.A. Pople, J. Chem. Phys. 93 (1990) 2537. [13] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 90 (1989) 2154. [14] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 94 (1990) 5523. [15] R.A. Alberty and R.S. Silbey, Physical Chemistry, John Wiley and Sons, New York, 1992, ch. 19. [16] I.N. Levine, Physical Chemistry, 3rd ed., McGraw-Hill, Singapore, 1988, ch. 17. [17] L.A. Curtiss, J.E. Carpenter, K. Raghavachari, J.A. Pople, J. Chem. Phys. 96 (1992) 9093. [18] W.J. Hehre, L. Radom, P. von R. Schleyer and J.A. Pople, Ab initio Molecular Orbital Theory, Wiley-lnterscience, New York, 1985, ch. 6.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 270 (1997) 363-368
A Gaussian-2 molecular orbital study of the unimolecular reactions of acetyl cyanide Suk Ping So Chemistry Department, The Chinese Universityof Hong Kong, Shatin, N.T., Hong Kong
Received 21 November1996;in final form 20 March 1997
Abstract
Some possible unimolecular reactions of CHaC(O)CN in its ground electronic state have been studied with the G2 molecular orbital method. Among the reactions considered, only its decomposition to CO and CH3CN is exothermic (A H = -7.66 kcat mol-I) and its isomerization to CH 3C(O)NChas an activation barrier (40.77 kcal mol-I) accessible at low temperature. CH3C(O)CN is predicted to be rather stable at room temperature in line with experiment since the former reaction channel and the second step of the latter channel by which CH3C(O)NCdecomposes to CO and CH3NCboth have very high barriers, viz. 109.47 and 100.31 kcal mol-1, respectively.
1. Introduction Formyl cyanide HC(O)CN is a molecule of considerable chemical interest. It is the simplest prototype of the acyl cyanides formed from an aldehyde and a cyano group. In addition, it has various potential uses as a formylating agent for nucleophilic species in organic syntheses. Although there were several early attempts to synthesize formyl cyanide, evidence for its existence was not reported until 1986 [1]. This is partly attributed to its thermal instability which has been studied by different experiments [1-4]. Theoretically, ab initio molecular orbital studies on the various possible unimolecular reaction paths (analogous to reactions la, lb and 3 - 6 shown below) of HC(O)CN have been reported recently [5,6]. Results suggest that HC(O)CN is rather stable at room temperature but can decompose into HCN and CO at high temperature. Acetyl cyanide (or pyruvonitrile) CH3C(O)CN is the second member of the acyl cyanide family. Ben-
nett et al. [7] investigated its pyrolysis at 470°C in a flow system. They found that the reaction proceeded competitively via channels 1 and 2: CH3C(O)CN ~ CO + CH3CN,
(la)
CH3C(O)CN ~ CO + CH3NC,
(lb)
CH3C(O)CN ~ CH 2 = C O + HCN.
(2)
Recently, Okada and Saito [8] studied the thermal unimolecular reaction of CH3C(O)CN diluted in Ar behind reflected shock waves over the temperature range of 1014-1300 K. It should be noted that both groups of researchers did not differentiate reaction la from reaction lb. Contrary to the result of Bennett et al. [7], methyl cyanide (or acetonitrile)CH3CN, one of the products of reaction 1, was not found by Okada and Saito [8] in their shock wave study of the unimolecular reaction of acetyl cyanide. They also performed an ab
0009-2614/97/$17.00 Copyright© 1997Elsevier Science B.V. All rights reserved. PH S0009-261 4(97)00367-9
364
S.P. So/Chemical Physics Letters 270 (1997) 363-368
initio molecular orbital calculation on the reaction via paths 1, 2 and 3. CH3C(O)CN ~ CH3C(O)NC.
(3)
The barriers (activation energies) to the reactions 1, 2 and 3 were computed to be 114.1, 78.4 and 58.5 kcal mol -~ at the H F / 3 - 2 1 G + Z P E level. This indicates that unimolecular decompositions of acetyl cyanide are energetically less favourable, and its isomefization to acetyl isocyanide CH3C(O)NC is dominant. The above theoretical study of Okada and Saito [8] on CH3C(O)CN was done only at a rather low level of theory (HF/3-21G) and only three reaction paths were considered. In addition, results [4,5] for HC(O)CN have shown that the energy barriers and geometries, particularly for the transition state structures, differ substantially with the level of theory employed. Accordingly, it is thought desirable to investigate, at a higher level of theory, namely the Gaussian-2 theory, the unimolecular reaction of CH3C(O)CN for the following reaction paths in addition to those mentioned above. CH3C(O)NC ~ CO + CH3NC, CH3C(O)CN ~ CH3OCCN , CH3C(O)CN ~ CH3COCN.
(4) (5) (6)
2. Calculations
The equilibrium and the transition state structures involved in the reactions studied were optimized by the energy gradient method at the HF and the MP2(FU) levels (MP2(FU) denotes MP2 theory with full electron correlation, i.e. including inner shell electrons), using the GAUSSIAN 94 programs [9] implemented on our IBM RS/6000 (Models 320H and 390) and SGI R8000 workstations. The energies of the above optimized stationary point structures were computed at the Gaussian-2 (G2) theory [10] which is the improved Gaussian-I (GI) theory [11,12]. Computationally, the G2 procedure involves the following steps: (a) Geometry optimization at the HF/6-31G* and the MP2(FU)/6-31G* levels;
(b) Calculation of the H F / 6 - 3 1 G * / / H F / 6 31G* vibrational frequencies; (c) Single-point calculation, for the MP2(FU)/631G* geometry, of frozen-core MP4SDTQ and QCISD(T) energies with the 6-311G** basis, MP4SDTQ energies with the 6-311 + G * * and 6311G(2df, p) bases, and MP2 energies with the 6-311 + G(3df,2p) basis. The G2 energy is defined [10] as the G1 energy plus a correction term (A) which eliminates a number of deficiencies present in the G1 theory. The G1 energy [11,12] on the other hand, is just the frozencore M P 4 S D T Q / 6 - 3 1 1 G * * / / M P 2 ( F U ) / 6 - 3 1 G * energy with various corrections. These include a correction ( A E ( + ) ) for the diffuse functions, a correction (AE(2df)) for the higher polarization functions on non-hydrogen atoms, a correction (AE(QCI)) for the correlation effect beyond the fourth-order perturbation theory, a 'higher level correction' (A E(HLC)) for the remaining basis set deficiencies (G1 and G2 have different AE(HLC) values), and the zero-point vibrational energy (ZPE); or, equivalently, the GI energy is the approximate energy of a full calculation at the frozen-core QCISD(T)/6-311 + G(Zdf,p)//MP2(FU)/6-31G* level plus the A E(HLC) and the ZPE corrections. The connection between each transition state structure and the reactant and the product(s) was established by intrinsic reaction coordinate (IRC) calculations based on the reaction path following algorithm of Gonzalez and Schlegel [ 13,14] as coded in Gaussian 94, or by optimization starting from a transition state structure with one or two of its geometrical parameters distorted.
3. Results and discussion The various stationary point structures studied are depicted in Figs. 1-3 together with their optimized geometrical parameters. These structures have been confirmed to be either equilibrium structures (1, 2, 3 and 4) or transition state structures (TSIa, TS3, TS4, TS5 and TS6) by computing their HF/6-31G* vibrational frequencies. Okada and Saito [8] have calculated the structures of CH3C(O)CN 2 and the transition states TSla, TS2 and TS3 for the reactions l a, 2 and 3, respec-
S.P. So/Chemical Physics Letters 270 (1997) 363-368 He
Hc
i.oeo\
(1~o>,, ~ /" .01~
H
1.0~0 H 10~1 <,=,'x, 1~0 1.079 ~¢6 (,~,.o>~),.;.,,'.~. 1.083
',N1
1•
125.6 (125.2) f ' - - " . / 1.503 ~ll, 4 '.,_ ,.." (1.199) " O--~Ca
,.,,3
lu.J L/
k...\ ,.49o
(149~
(1.211) 120.3~(1.425) 1.1~ (1~0.2) M . , ~ IZIJ N
(I.12~) 119-%\ (I.,tg0) (119.5) ~\ 178.3 (~C (i ~0.2)~ , . ~ I.I ~d. (1,1tl2)
I~.~(~\
(179.4) \ 17&9 \
L CaCbH¢=109.2(109.l)109.4 / CtCbH=I09.8(I09.7)I09.6 I
OCaCbH---~:120.9(+121.0):t120,8
2 (Cs)
1 (Cs) /
1166
c
N L CiCbHc= 109.2( 109. I ) / C,CbHffil09.9(109.9) L OC,Cbl I---~:120.9(_+ 121.0)
H
LI~
(1.19~)
i.trm (1:'~9) i06,5 (l.09a) / C bq' ' ~ " *
f
.
1.42
(114.6)
1,341
o,
o.~71o
1.276~.. \ 1.471 (13041 ~(1.41~)
'°',~k ~'~ (168.O 1,2.3 (106.9) N
3 (C s)
,~c/
(i.4u)
11.09"/) .l~t3
o.~97>
i~ f..-; (,~_~/ <1,',,%'
r,6.4 /'~
075.0) I, " v
(LI~X / OCbH,=109.4(108.6) L OC~H=107.4( 106,7 ) / C ~ O C b l l ~ 120.9(_+ 121.0)
(104.9> 1 1 . 4 8 1 1 " H d
"N/~('1~11~)
L CtCbttc=i 15.4(116.0) / CaCbHd=108.3(106.8)
365
its triple-bond character (1.178 ,~, in CH3CN) during the complete course of the reactions considered. The dissociation reactions of CH3C(O)CN and CH3C(O)NC studied in this work are represented by reactions la, lb, 2 and 4. In contrast to the case of HC(O)CN, attempts to locate the transition state for the reaction lb of CH3C(O)CN have not been successful in this work. The optimized transition state structures T S l a , TS2 and TS4 for the corresponding reactions l a, 2 and 4 are depicted in Fig. 2. As anticipated, the 3-21G geometrical parameters of T S l a and TS2 obtained by Okada and Saito [8] deviate significantly from the values predicted here. Fig. 2 shows that both T S l a and TS4 contain a loose three-membered ring each, while TS2 a loose fourmembered ring comprised of CCCH. The transition state structures T S l a and TS2 are of C~ symmetry, but TS4 becomes C 1 with the C atom of the NC
Z_ C,~CbH~=106.1(106.7) / 0C,CBH¢=+8.7(0.0) L OCaCbt Id =+ 133.7(+ 124.0 )
/ OCaCbH~=-] 13,0(-t23.9) 4 (C1)
I •I09
Ha
2.253
°---rc.,..... ::L. Ig~,'--".9oo,
/~.~ ':~--
Fig. 1. Optimized HF/6-31G * and MP2(FU)/6-31G * (in parentheses) equilibrium structures of isomeric C 3NOH 3. Bond lengths are in angstroms and bond angles in degrees. HF/3-21G values for 2 from Refi [8] are in italics.
'1~'/
tively, at the H F / 3 - 2 1 G level of theory. These are included in Figs. 1-3 for comparison purpose. It is found that their 3-21G geometry of 2 agrees quite well with the present 6-31G * value, indicating that this geometry is rather insensitive to the size of the basis set. However, the agreement is much poorer for the transition state structures T S l a , TS2 and TS3. It should be noted, as pointed out above, that Okada and Saito [8] did not differentiate reaction la from reaction lb, and so did not differentiate the corresponding transition states T S l a and TSIb. The length of the CN bond of the structures shown in Figs. 1-3 is seen to lie between 1.181 and 1.193 ,~ at the MP2(FU)/6-31G* level except those of the transition states TS2, TS3 and TS4, which are o somewhat longer, viz. 1.198, 1.205 and 1.211 A, respectively. This insignificant variation of the CN bond distance between the structures studied, as also noted in the case of HC(O)CN [6], indicates the difficulty of breaking the CN bond, which preserves
L CaCbH=104.3(113.9)117.6
".,~2 .." " ~ " 1 ~
2.692 ; ,." 2.523 19.419> '. , / (2.256> 2.5.tl (',co • 2,412 153.7 ~ , (143.8) 1.150 143.3 ( . 92) N 1335
(1.0~1) 1,066
/" C,CBH¢=81.7(85,8182.9 L CCaCbH=:I:62.1X+67.0)_+62.7
TSla (Cs) L109 0.148)
1,421 (1A24)
1.103 (1,142)
I.II4 1.432 O----~C,--~-----C~
109.6
I-I 1,082 ~ , (I ,093) f g--
;
(111.3)k--~-;,~.s 1o1~,~ i (,~2 (2.795) :
.
1.083 H
~'"
(2.4101 ;
:<115,>., i1. .H e
(LI9g) 1.155
L CjCbHc=100.0(101,0)96.2 L C,C~H=109.5(109.0)110.7 L CCaCbl t ----L-_117.6(+ 118.7)+_117.1
TS2 (Cs)
Hc I" 1I'668 I (
(io~.o> '.~9.s ~7 : .c,,c::~ ....
,.~
2.761 ; .-*° 9.1.3 ~ . +- "" I.,56 (71,1) (~.C'" (1,769) 139.0N~52 I,&T7
2.365
o ,,-77~ ~. .. . . .(21~,> 111.2 ; ....... . , ;
. " 2.494 o°" ('2.323) °* ' / 122.5 N~k)(118.2)
Hd
I.t'~ (1.076)
1,166 "% (1.211> C
/ ' CaCbHc=78.9(82.6) / - CaCbtIn=105.3( 125.4) ~-- CaCbHo=100.0(93.8)
i L L L
OCaO_~t-lc=+165.7(+ 142.9) OCaCbttd=+46.8(+38.41 OCaCbH¢=-75.8(-100.21 CNCbCa=-97.6(-96.0)
TS4 (CI)
Fig. 2. Optimized HF/6-31G * and MP2(FU)/6-31G * (in parentheses) transition state structures for the decomposition reactions of CH3C(O)CN and CH3C(O)NC. Bond lengths are in angstroms and bond angles in degrees. HF/3-21G values for T S l a from Ref. [8] are in italics,
366
S.P. So / Chemical Physics Letters 270 (1997) 3 6 3 - 3 6 8 He I.[~1
1.~2
(i.091)~ 132.0
1.152
Cb.~.,. ~ Ha
(133.7) / ,
o.lsu
09 (l'~i) HQ
i~s6 / 2 " ~ ,
I.IJ7
lzs2
ll.q~j
I 092
I" CaC1~1~=108.4(109.0)109.2 / CaCbHd=108.9( 108.3)108.9
/ CaC~rI¢=I09.4(108.9)I08.9
( '~.':" I ,.','~o I
,.~o
/ / L /
1.925
(1.871) ,fi" ('/0.6) ,(1.886) 1.954 ** 6
67~
2.078
•
OCaCbHd=+120.5(+ 121.0)+120.7 OCaCbHc=-121.0(-121.4)-121.3 NCaCbI-Ie=+155.6(+ 157.8) CC,CbHc=- 165.9(- 163.0)
(i.2os)
t.tzz
T S 3 (CI)
He 1.07$~ Ho 1.1")69 (1.094) " ~ ~ (I.(~l) ,' ',""~DNn 1.0"79 / ",, u.097)
He I.~5~ ~ O.0%) Cb(,~
I ~
1.g43 : ". 85, (I.724)" 696". (1.795)
2.g2(111520~, / i1'47~9 C~1;324) /Z:-~J (. 9)
. (65.3)C', O--~C,
\
(I.313) 1.506 1.251 (I13.1) 113.5{~llI (1.459) ~- CaCbH =120.5(125 5"~
,'c,cb.~=9o0(s837"
/-- CaCbHc=101-2(103.4) / _ OC.CbH¢=+31.0(+31.2)
|'~
1.1~6 O.095) H
1.415 '~~. (I.452)
172,4
(55.6) ," ,"(I.704) 1.613
~'/
157.5
V ('~'5, <,,:,,;~>i ~`'5'~> 1.138~
0.184)~
N
/ OCaCI~Id=+ 145.9(+ 141.4) / OCaCt, H,~=-97.8(-103.1) /-- CCaOCb=+I 12.3(+112.7) L NCC~,O=+176.8(+ 164.5)
L CaCbHc=I 11.2(I I 1.4) . i CaCbtl=108.4(108.3) / OCaCbIl--!--122.1 (+ 122.4)
T S 5 (CI) Fig. 3. O p t i m i z e d H F / 6 - 3 1 G
T S 6 (Cs) * and MP2(FU)/6-31G
* (in paren-
t h e s e s ) transition state s t r u c t u r e s f o r the i s o m e r i z a t i o n r e a c t i o n s o f CH3C(O)CN.
B o n d l e n g t h s are in a n g s t r o m s and b o n d a n g l e s in
degrees. HF/3-21G
v a l u e s f o r T S 3 f r o m R e f . [8] are in italics.
bond sticking out of the plane of the non-hydrogen skeleton (referred to as molecular plane hereafter). Acetyl cyanide has three isomerization pathways. In reaction 3, the migration of the acetyl group from the C atom to the N atom of the CN group results, via the transition state TS3, in the isomer CH3C(O)NC which may decompose into CO and CH3NC, as in the case of HC(O)CN [5,6]. The 1,2-CH 3 and the 1,2-CN shifts from the C atom to the O atom lead, via the transition states TS5 and TS6, to the products CH3OCCN (reaction 5) and CH3COCN (reaction 6), respectively. The transition state TS3 may be taken from Fig. 3 as arising from the migration of the N atom and the twisting of the CN bond out of the molecular plane. Its MP2/631G* CaC and CaN bond distances are 1.871 and 1.886 A, respectively. Further enhancement of the CN interaction and the breaking of the CaC bond lead to the product CH3C(O)NC. When isomerizing to CH3OCCN , CH3C(O)C N goes through the transition state TS5 which involves not only the migration but also the rotation of the CH 3 group and the bulging of the CN bond out of the molecular plane. The isomerization of CH3C(O)CN to CH3COCN , on the other hand, results from an in-plane 1,2-CN shift. The three transition state structures described above involve a loose three-membered ring each (Fig. 3).
Table I E n e r g y c o m p o n e n t s a n d G 2 total e n e r g i e s Species
E(MP4SDTQ/6-311G*
*) a
AE(+)
b
A E(2df) b
AE(QCI)
t,
A b
AE(HLC)
b
Z P E b.c
E(G2) a -245.688337
1
-245.531872
-9.335
- 128.365
+7.913
- 14.632
-65.000
+52.954
2
- 245.509055
- 8.978
- 128.532
+ 6.240
- 15.385
- 65.000
+ 52.440
- 245.668270
3
-245.429668
- 10.548
- 128.178
+4.492
- 14.877
-65.000
+52.474
-245.591305 -245.568291
4
-245.403017
- 10.051
- 130.086
+3.144
- 14.000
-65.000
+50.721
TSIa
-245.349146
- 11.164
- 127.005
+6.079
- 14.178
-65.000
+46.536
-245.513878
TS2
-245.423710
- 12.289
- 128.149
+ 10.736
- 14.849
-65.000
+45.764
-245.587497 -245.623369
TS3
-245.464355
- 10.191
- 126.761
+7.761
- 15.181
-65.000
+50.358
TS4
-245.346625
- 10.895
- 125.384
+8.734
- 15.069
-65.000
+45.815
-245.508424
TS5
- 245.349202
- 11.279
- 129.539
+4.999
- 15.269
- 65.000
+49.582
-245.515708
TS6
-245.364547
- 10.501
- 131.165
+7.061
- 14.915
-65.000
+49.415
CO
- 113.098623
-3.720
-54.202
+5.060
-5.974
-25.000
+4.962
CH3CN
- 132.448614
-3.769
-68.091
+3.331
-9.564
-40.000
+43.666
- 132.523041
CH3NC
- 132.409025
-4.258
-67.731
+2.336
- 10.425
-40.000
+43.505
- 132.485598
-5.151
-80.998
+4.957
-9.835
-40.000
+30.542
- 152.369110
- 3.008
- 46.991
+ 3.008
- 5.124
- 25.000
+ 16.062
- 93.284894
CH2 =COHCN
152.268625 - 93.223841
a In hartrees. b In m i l l i h a r t r e e s . c HF/6-31G
* f r e q u e n c i e s ( e x c l u d i n g i m a g i n a r y o n e s ) s c a l e d b y 0.893 are u s e d to calculate Z P E [12].
-245.529652 - 113.177497
S.P. So / Chemical Physics Letters 270 (1997) 363-368
The energy components and the G2 total energies of the various species studied are listed in Table 1, and the potential energy surface for the unimolecular reactions of CH3C(O)CN is depicted in Fig. 4. It is seen that, at the G2 level of theory, CH3C(O)NC, CH3OCCN, and CH3COCN lie 12.69, 60.89 and 75.33 kcal m o l - ~ higher in energy than CH 3C(O)CN, respectively. This order of relative stability has also been predicted for the case of HC(O)CN [5]. As shown in Fig. 4, among all the decomposition and isomerization reactions considered, the isomerization of CH3C(O)CN to CH3C(O)NC (reaction 3) has the lowest barrier, as in the case of HC(O)CN [5], and is the only reaction feasible at low temperature because of its low barrier of 40.77 kcal mol-1 The other two isomerization reactions 5 and 6 have much larger barriers (99.58 and 108.33 kcal mol-1 respectively) and are thus likely to occur only at very high temperature. That is, with these activation energies, reactions 3, 5 and 6 would have some conveniently measurable specific rate k of, say, 1.5 × 10 .2 s- i when carried out at 600, 1470 and 1600 K, if the pre-exponential factor of Arrhenius equation is set
Potential Energy (kcal/mol)
120.0
~
TS4
TSla
100.0
lL290
TS5 l(f,) 47
r s 6 10g.33 99.58
80.0
H~COCN
60.0
4 75.33
60.~9
TS3 40.0 HCN + 2:0-0
2154 12.39
O.O
co. (
1 oo "
-7.66
Reaction Coordinate
Fig. 4. Semi-quantitative potential energy surface for the unimolecular reactions of CH3C(O)CNcalculated at the G2 level of theory.
367
equal to a median value of 1013 S-t usually found for unimolecular reactions [15,16]. The decomposition of CH3C(O)CN to CO and CH3CN (reaction la) is exothermic by 7.66 kcal moi -1 at the G2 level, yet it is kinetically unfavoured owing to its large barrier of 109.47 kcal mol - l . This is similar to the case of HC(O)CN [6] and is in line with the experimental observation [8] that no acetonitrile CH3CN, one of the products of reaction la, has been detected in the thermal unimolecular reaction of CH3C(O)CN diluted in Ar studied behind reflected shock waves over the termperature range of 1014-1300 K. The decomposition reaction 2 of CH3C(O)CN to HCN and CH 2 = C O is predicted in this work to have an intermediate activation energy of 63.3 kcal moland hence to be feasible at moderately high temperature. The decomposition channel of CH3C(O)CN to CH3NC and CO via the isomeric intermediate CH3C(O)NC which lies 12.59 kcal mol-• higher in energy consists of two successive steps, i.e. reactions 3 and 4 (Fig. 4). Their respective activation energies are 40.77 and 100.31 kcal mol -~, each of which is lower than those of reactions la (109.47 kcal mol- ~). The barrier for CH3C(O)NC to revert back to CH3C(O)CN is only 28.18 kcal tool -t which is significantly lower than that (100.31 kcal tool -l ) for CH3C(O)NC to decompose to CH3NC and CO. This t w o - step reaction channel may be viewed as having a pseudo barrier of about 112.90 kcal mol -~ (Fig. 4) and, only at high temperatures, can be regarded as having the second step, i.e. reaction 4, become the rate-determining step with a barrier of 100.31 kcal m o l - t . Similar results have been predicted for HC(O)CN [5]. From their IR emission data, Okada and Saito [8] obtained a A H value of 5 kcal mol -I for the isomerization (reaction 3) of CH3C(O)CN to CH3C(O)NC, which is in excellent agreement with their H F / 3 - 2 1 G energy difference of 5.2 kcal molbetween CH3C(O)CN and CH3C(O)NC. However, the corresponding value calculated in this work at the G2 level is more than twice as large, viz. 12.59 kcal mol -~. This thus casts some doubt on the reported IR emission data because G2 values are believed to be more reliable than H F / 3 - 2 1 G values in view of the small average absolute error (around
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1.2 kcal mol - l ) of G2 energy differences and the low level of theory and limited basis set of the HF/3-21G method [17,18].
Acknowledgements The author thanks the Hong Kong Research Grant Council for an Earmarked grant (RGC REF. CUHK 72/92E) and the Chinese University Research Committee for a Direct Grant to support fully and partially the acquisition of the RS/6000 Model 390 and the SGI R8000 workstations, respectively. The assistance of Mr. Philip Ling of the Chinese University Computer Services Centre in implementing the Gaussian 94 package on the workstations and in taking care of system problems is appreciated.
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