1. 5065
Table I. Relative Intensities of Selected Ions”
C&sCHzCOzCHzC&s C&,~CDZCOZCDZC&~~
C&sCHzCOzCDzCsHsb C&sCDzCOzCHzC&sb
(1) (11) (111) (IV)
M.+ loOC 1O O C lOOC 1ooc
C7HsD2 324 196 102
C7HGD1 21.7 8.4 14.6
C7H7 319 13.0 110 191
CsH3Dz 18.7 9.12 6.99
CSHIDI 20.1 9.98 8.25
CjHs 41.4 8.1 21.1 21.2
a Corrected for contributions. I1 was found to be 88.8 atom % d4,9.5 atom % d3,and 1.7 atom d2; 111 was found to be 97.7
atom % dz and 2.3 atom % dl; IV was found to be 95.4 atom dz and 4.6 atom % dl by low-voltage mass spectrometry. c Total ion inten-
sity for all molecular ions, including partially deuterated molecular ions.
Meyerson’ for bibenzyl-a,p-d4 (dz = 0.40; dl = 0.43; include ion kinetic energy measurements and low-
do = 0.165), which is thought to experience complete voltage spectra will be presented in the full paper.
H-D randomization in the decomposition of the tropyl- Acknowledgment. Acknowledgment of financial sup-
ium ion. However, a tropylium ion formed by random port of this work is made to the Kansas State University
insertion should produce the C5H5+, C5H4D+, C5H3-
and Bureau of General Research and (by J. C. W.) to the
D2+ions in the ratio 0.47:0.47:.047 for 11, values National Science Foundation Undergraduate Research
substantially different from those found (0.40 :0.43 : Participation Program, Kansas State University, sum-
0.17). mer, 1972.
T o better visualize what the data may mean, the
isotopic distributions of C5H5+ions from C7H7+ions Michael K. Hoffman,* James C. Wallace
were calculated assuming that the C7H7+ions from eq Department of Chemistry, Kansas State University
1 and 2 are similar in structure and have completely Manhattan, Kansas 66502
randomized H-D prior to loss of acetylene. The Receiaed December 4, 1972
results of these calculations, expressed as observed/
calculated ratios, are given in Table 11.
Geometrical Reaction Coordinates.’ 11.
Nucleophilic Addition to a Carbonyl Group
Table 1 . Observed/Calculated Ratios for Distribution
1
of Deuterium in C j Ionsa Sir :
I1 I11 IV In the course of a survey of crystal-structure analyses
~~
d2 0 93 0 76 1 22 of compounds containing nucleophilic groups and
di 0 91 0 78 1 18 electrophilic centers we have discovered some struc-
do 1 79 1 36 0 90 tural regularities which, we believe, provide an experi-
A referee has suggested this form for presentation of data. mental basis for mapping the reaction coordinate (min-
imum energy pathway) for the addition reaction of a
nucleophile to a carbonyl group. The observed dis-
The data are clearly inconsistent with complete tance between nucleophile (N:) and carbonyl c$rbon
randomization of H-D within the decomposing “C7H7” ranges, in the examples $udied, from about 1.5 A (co-
ions. Indeed, the scrambling results are not consis- valent bonding) to 3 A ; the longer distances have
tent with C, models of benzyl-a-dz, tropylium-1,2-dz, sometimes been interpreted as evidence for nonbonded
or norcaradienyl-d2, if equal rates of decomposition interactions of donor-acceptor or dipole-dipole type.
and similar structure of the two C7 moieties are as- We take the view that each example can be regarded as
sumed. a case where the addition reaction has proceeded to a
The ratios in Table I1 likewise indicate that the C7 greater or lesser extent but has been frozen in at a cer-
ions derived from the benzyloxy grouping decompose tain stage by the intra- or intermolecular constraints
less readily than those formed from the phenylacetate imposed by the crystal environment. Each case then
benzyl portion. This result, in part, may account for provides a sample point on or close to the reaction co-
the greater intensity of the C7 ion formed by benzyl- ordinate, and a sufficiently large number of points
oxygen bond cleavage. should map out the reaction coordinate, provided that
The apparent difference in the rate of decomposition the perturbations from the different crystal and molec-
of the C7 ions is most readily explainable if the reactive ular environments can be averaged out. Similar ar-
C7 ions formed by the two processes are structurally guments1,2have been used to derive the minimum en-
different. The labeled benzyl phenylacetates thus ergy pathway for the nucleophilic substitution reac-
represent the first case in which decomposing C7H7+ tions I- +
1-1 + [ I . . . I . .I]- + 1-1 I- and X + +
+
t
ions have not attained an equilibrated energy mani- CdS3Y + [ X . . .CdS,.. .Y] + XCdS, Y. In this
fold on the normal ion time scale.1° Further studies communication we present results for the addition of
t o elucidate the exact structure of the two C7H7 ions an amino group t o a carbonyl or, equivalently, for the
(IO) An ion kinetic energy study of isomeric C7H7 methyl ethers in-
reverse elimination process.
dicates equilibrium to a common structure occurs in the formation Structural parameters for intramolecular N . . . C=O
of the C7H7+ ion. GH7+ ions which react further, however, may do so
from structurally different forms on the metastable time scale: R. G. (1) Part I : H . B. Burgi, Inorg. Chenz., in press.
Cooks, M. Bertrand, and M. K. Hoffman, manuscript in preparation. ( 2 ) H. Bent, Chem. Rec., 68, 587 (1968).
Communications to the Editor

2. 5066
Table I. Experimental Structural Parameters for N . . .C=O Interactionsa,*
Compound 11. 1%. di d2 A (Ye P1 P2 P 3 P 4 -p Ref
A, methadone 1.510 1.554 2.910 1.214 0.064 105.0 119.7 122.2 89.7 82.7 117.5 4a
B, cryptopine 1.521 1.515 2.581 1.209 0.102 102.2 121.2 119.0 85.0 95.2 118.1 4b
C, protopine 1.520 1.505 2.555 1.218 0.115 101.6 120.8 120.0 84.7 97.7 117.2 5
D, clivorine 1.508 1.546 1.993 1.258 0.213 110.2 118.9 115.7 92.0 93.6 118.8 6
E, retusamine 1.49 1.56 1.64 1.38 0.36 110.9 115.7 107.0 101.4 100.1 120.2 7
F, N-brosylmitomycinA 1.56 1.56 1.49 1.37 0.42 113.7 103.6 113.6 102.0 103.9 119 9
-
8
aFor a definition of symbols see Figure 1. Distances in bngstroms, angles in degrees; ~ ( l ) ~ ( d_< 0.02
1.3quot;. (Li) = 1.529 A, (a)= 107.3 (5.0)quot;, (7) = 118.6 (1.2)quot;.
) A; u((Y) - ~ ( p-
) a(?)
~
5
P
1
I I
1 0.3
I
i
-02
c
-
o.2
0.1
1.5 2.0 2.5
Figure 1. (a) Definition of symbols. (b) Reaction coordinate Figure 2. Correlation plot of A us. dl (smooth curve obtained from
projected on the NCO plane showing nitrogen (top, arrows in- eq 1).
dicate the estimated direction of the lone pair), carbonyl oxygen
(bottom right), and bisector of y (RCR') (bottom left),
reproduce the obseEved distances dl and dz with a mean
interactions3 (Figure la) obtained from six crystal-struc- deviation of 0.03 A and hence imply that the sum of
ture analyses4+ are summarized in Table I. In all the C-0 and C-N bond numbers equals two for all the
cases the N, C, and 0 atoms lie in an approximate local structures under discussion.
mirror plane (m) but the C atom deviates significantly It is difficult to detect any systematic trend in the
from the plane defined by R, R', and 0; the amount of angles cy and y , which do not vary by more than a few
this deviation (A) increases as the C . . N distance (dl) degrees from their mean values. For the subsequent
decreases. From Figure 2 it is seen that the experi- discussion they are assumed to be constant.
mental points lie close to the smooth curve The reaction coordinate, projected on the NCO
plane, can be followed from the three sets of points
d1 = - 1.701 log A + 0.867 A A-F shown in Figure Ib. As the nucleophile ap-
where the constants were found by least-squares anal- proaches the carbonyl C atom, the two alkyl substit-
uents R and R ' bend away and the C-0 distance be-
ysis. Extrapolation to dl = 1.479 A, the standard
comes slightly longer. The virtual constancy of a
C-N bond length,g leads to A,,, = 0.437 8, and eq 1
shows that the nucleophile approaches along a line.
can be rewritten as
This line is not perpendicular to the C-0 bond but
dl = -1.701 log n + 1.479 8, (2) forms an angle of about 107quot; with it. We find approxi-
mately the same value of cy from parallel studies with
with n = AJA,,,. A similar equation (3) can be given other nucleophilesquot; Although we cannot observe the
dz = -0.71 log (2 - E) + 1.426 A
(3) direction of the lone-pair orbital on the nitrogen atom
it can safely be assumed to lie close to the local three-
for the C-0 distances; here the multiplicative constant fold axis of the tertiary amino group. From the ex-
is taken from Pauling's equation relating C-C bond perimental data, this direction coincides with the
length and bond number, lo and the additive constant is N C direction within a few degrees, except in the
s +
the standard C-0 bond distance.9 Equations 2 and 3 case of methadone where the distance is largest and the
(3) All recent work on transannular N . * .C=O interactions owes
angle is about 21quot;. From this we conclude that there
much to the early investigations of Sir Robert Robinson and N. J. is a strongly preferred orientation for nucleophilic
Leonard. For an account of the early chemical and spectroscopic attack, as has been postulated from analysis of kinetic
studies in this field see N. J. Leonard, Rec. Chem. Progr., 17, 243 data. l 2
(1956), and references cited therein.
(4) (a) H. B. Burgi, J. D. Dunitz, and E. Shefter, Nature (London), As the RR'C=O system becomes nonplanar the T
in press; (b) S. R. Hall and F. R . Ahmed, A c f a Crystallogr., Sect. B, and T* orbitals of the carbonyl group develop into a
24, 346 (1968).
( 5 ) S . R. Hall and F. R . Ahmed, Acta Crystallogr., Sect. B, 24, lone-pair orbital on oxygen and an empty spquot;-type
337 (1968). orbital on carbon. The closer the approach of the
(6) K. B. Birnbaum, Acta Crystallogr., Sect. B, 28, 2825 (1972). nucleophile, the larger the nonplanarity of the RR 'C=
(7) J. A. Wunderlich, Acfa Crystallogr., Sect. B, 23, 846 (1967).
(8) A . Tulinsky and J. H. van der Hende, J . Amer. Chem. Soc., 89, 0 system, the higher the s character of the empty orbital,
2905 (1967).
(9) L. E. Sutton, Chem. Soc., Spec. Pub!., No. 18 (1965). (1 1) H. B. Burgi, J. D. Dunitz, and E. Shefter, unpublished results.
(lo) L. Pauling, quot;The Nature of the Chemical Bond,quot; 3rd ed, Cornell (12) D . R. Storm and D. E. Koshland, Jr., J . Amer. Chem. Soc., 94,
University Press, Ithaca, N. Y . , 1963, p 246. 5805, 5815 (1972).
Journal of the American Chemical Society 9S:lS 1 July 25, 1973

3. 5067
the lower its orbital energy, and the greater its interac- Table I. Calculated and Experimental Vertical Excitation
tion with the nitrogen lone pair. In the initial stages, Energies for Selected ?r-Electron States of
trans-Butadiene (in eV)
where the reaction coordinate is not controlled by
orbital overlap considerations, the main interaction -____ Excitation energies------
would appear to be between the local electric dipoles State Calcd Optical Electron impactquot;
of the nucleophilic and the carbonyl groups. These Valence Single-Excitation States
conclusions are somewhat analogous to the interplay 1 3 ~ quot; 3.45 3 . 22b 3.20
of charge polarization and orbital overlap occurring in 13A, 5.04 (3.909 4.91
the course of SN-type reactions, according to recent Valence Double-Excitation States
SCF-MO calculations. l 3 2IA, 6.77
3 3BUc 8.08
(13) J. P. Lowe,J. Amer. Chem. SOC., 93,301 (1971). 1 5 ~ , 9.61
(14) G . A. Pfeiffer Fellow (1972-1973) of the American Foundation
for Pharmaceutical Education; on leave from the School of Pharmacy, Diffuse (Single-Excitation) States
State University of New York at Buffalo. 1'B, 7.05 6.056 5.92
2lB, 8.06 27.06e 7.28
H. B. Biirgi, J. D. Dunitz,* Eli Shefter14 3 'A, 7.82
Laboratories for Inorganic and Organic Chemistry 41A, 7.87
Federal Institute of Technology, Zurich, Switzerland
Received April 30, I973 Reference 7. D. F. Evans, J. Chem. Soc., 1735 (1960); the
3.9 eV state has not been confirmed by other data and is considered
spurious. c The z3Bustate is a Rydberg state and has a calculated
excitation energy of 7.14 eV. The calculated excitation energies
Low-Lying n-Electron States of trans-Butadiene for the diffuse states are believed too high by about 1 eV, see text.
e W. C. Price and A. D. Walsh, Proc. Roy. Soc., Ser. A, 174, 220
Sir : (1940). The 7.05 eV state was assigned by R. S. Mulliken, J.
Chem. Phys., 7, 121, 373 (1939), as lA,.
A series of extensive configuration interaction cal-
culations on the 9-electron states of trans-butadiene has
been carried out and will be described in detail else- The two lowest triplet states, 13B, and 13Ag, are
where.' They were based on a self-consistent field found to be valence states with calculated vertical
representation of the u core, obtained from an all- excitation energies of 3.45 and 5.04 eV, respectively, in
electron SCF calculation on the ground state using a good agreement with recent electron impact's8 and ion
double-{ contracted Gaussian basis set (the SCF impactg spectra. Agreement is less good for the cor-
energy was -154.87712 hartree). In line with recent responding singlet states. In contrast to the assump-
calculations on ethylene3-+ in which the singlet ( ~ a * ) tions of a-electron theory and in line with the ab initio
state was found to have a diffuse charge distributioq6 results for planar the 1,2'B, and 3,41Ag
the 9 basis was augmented with two diffuse 2pa func- states of butadiene are found to be Rydberg-like in
tions (Gaussian exponents 0.03477 and 0.01075) on character (the 2'Ag state is of a difierent type and is dis-
each carbon atom. All the configurations which could cussed further below). While the u core obtained from
be constructed by distributing the four 9 electrons in all the ground-state SCF calculation appears to be adequate
possible ways between the 16 SCF occupied and virtual for the treatment of the low-lying valence ~ t a t e s such
,~
9 orbitals (2752 configurations for IAg, 3556 for 3Ag,
as 13B, and 1 3Ag,it is less appropriate for the diffuse
924 for 5Ag,2688 for lB,, and 3584 for 3B,) were in- Rydberg-like states; thus, in ethylene, relaxation of the
cluded in the configuration interaction treatment, which u core for the singlet ( a ~ * state (upon carrying out an
)
can thus be characterized as full n-electron CI with a SCF calculation for the excited state) lowered the com-
frozen SCF u core. puted excitation energy by 0.6 eV (but this effect was
The main results are summarized in Table I in terms smaller in recent CI calculations on butadiene with a
of the vertical excitation energies from the ground state smaller basis set by Shih, Buenker, and PeyerimhoffIo).
(the computed ground state CI energy was - 154.94015 Furthermore, u orbital correlation (neglected here and
hartree). The electronic states are characterized as by Shih, et al.) appears to affect the a ~ singlet states
*
either valence-like or diffuse (Rydberg-like), in terms of more than the ground state. 4 , l 1 The calculated vertical
the spatial extent of the wave function; no intermediate excitation energies are therefore expected to be too high,
cases have been found.' as indeed appears to be the case. The assignments
used for these states' in Table I were suggested inde-
(1) R . P. Hosteny, T. H. Dunning, Jr., R . R . Gilman, A. Pipano, and pendently by Shih, et a1.I0
I. Shavitt, to be submitted for publication. A preliminary report was In addition t o the states discussed above, three states
given at the 26th Symposium on Molecular Structure and Spectroscopy,
Columbus, Ohio, June 1971. with valence-like charge distributions were found with
(2) T.H . Dunning, Jr., J. Chem. Phys., 53, 2823 (1970). verticalexcitation energies of 6.67 (21Ag),8.08 (33B,), and
(3) T. H. Dunning, Jr., W. J. Hunt, and W. A. Goddard 111, Chem. 9.61 eV (15Ag). They could not be correlated with any
Phys. Lett., 4, 147 (1969).
(4) C. F. Bender, T. H. Dunning, Jr., H. F. Schaefer 111, W. A. spectroscopic states and were found to be predominantly
Goddard 111, and W. J. Hunt, Chem. Phys. Lett., 1 5 , 171 (1972). double-excitations relative to the ground-state Hartree-
(5) R . J. Buenker, S. D. Peyerimhoff, and W. E. Kammer, J . Chem.
Phys., 55, 814 (1971); R. J. Buenker, S. D. Peyerimhoff, and H. L. (7) 0. A. Mosher, W. M. Flicker, and A. Kuppermann, Chem. i'hys.
Hsu, Chem. Phys. Lett., 11, 65 (1971). Lett., 19, 332 (1973).
(6) It should be noted that the diffuse nature of the ( S T * ) singlet (8) H. H . Brongersma, J. A. van der Hart, and L. J. Oosterhoff in
state of planar ethylene has been questioned by a number of workers, quot;Fast Reactions and Primary Processes in Chemical Kinetics,quot; S.
notably H. Basch and V. McKoy, J . Chem. Phys., 53, 1628 (1970), and Claesson, Ed., Interscience, New York, N. Y., 1967, p 211.
J. A. Ryan and J. L. Whitten, Chem. Phys. Lerr., 15, 119 (1972), but the (9) J. H. Moore, Jr., J . Phys. Chem., 76, 1130 (1972).
results of the recent CI calculations of Bender, et al.,d and of H. Basch, (10) S. Shih, R. J. Buenker, and S. D. Peyerimhoff, Chem. Phys.
Chem. Phys. Lett., 19, 323 (1973), while indicating that electron correla- Lett., 16, 244 (1972); a comparison of the results of these authors with
tion has a significant effect on the charge distribution in this state, do those of ref 1 is given in their paper.
not support the objections of these authors. (11) K. Tanaka, I n t . J. Quantum Chem., 6, 1087 (1972).
Communications to the Editor