1482
The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
J.
ring is difficult. The corresponding 1,2-epoxy-3-butene does not appear to have been studied by either electron diffraction or microwave spectroscopy. An electron diffraction study of vinylcyclopropane" was interpreted to favor a mixture of 3 parts trans to 1 part gauche a t 293 K, which would correspond to a 6.5:l ratio at 204 K. Here, trans refers to the vinyl group trans to the ring and the gauche conformer is rotated -120' from trans. A microwave investigationI2 failed to uncover transitions belonging to another species. Also, as indicated in the Introduction, a number of related cyclopropyl derivatives have shown trans-cis conformers rather than trans-gauche. However, if accepted, the electron diffraction results predict a gauche-trans energy difference of approximately 1 kcal/mol, which is larger than the 0.37 kcal/mol difference determined for 3-methyl-1-butene, The corresponding aldehyde compounds can be intercompared because all three compounds have been studied. An electron diffraction study of 2-methylpropanal (isopropyl~arboxaldehyde)~ showed that 10% of the molecules have the oxygen atom trans to the isopropyl group (aldehyde hydrogen trans to isopropyl hydrogen) and -90% of the molecules are gauche conformers (at 266 K). Electron diffraction2 and microwave6 studies of cyclopropanecarboxaldehyde predict an approximately 50-50 of )species containing the oxygen atom mixture ( 4 5 7 ~ ~ 5 5 % cis and trans to the ring. Thus, the predominantly threefold potential in the isopropyl compound becomes a predominantly twofold potential when the methyl carbons are joined to form a cyclopropane ring. Finally, in a microwave study of gly~idaldehyde,~~ only species with the aldehyde oxygen approximately trans to the ring were found. It was concluded that any other species, if present, occur with considerably lower concentration. Thus, in the aldehyde compounds there is an increase in the relative stability of the trans species in going from isopropyl to cyclopropyl to ethylene oxide group. A corresponding increase in the relative stability of the trans species occurs for the vinyl compounds in going from the isopropyl to the
F. Stevens, R.
F. Curl, and P.
S.Engel
cyclopropyl group. Comparison of the two groups of molecules shows that the relative stability of the trans species is greater in the vinyl compounds than in the aldehydes. We do not a t present have a consistent interpretation of these interesting results.
Acknowledgment. One of the authors (R.H.S.) acknowledges with gratitude his introduction to the field of microwave spectroscopy and the subject of internal rotation in the laboratory of Professor E. B. Wilson. References and Notes The research in this paper was supported in part by grants from the National Science Foundation. L. S. Bartell and J. P. Guiliory, J . Chem. fhys., 43, 647 (1965). J. P. Guillory and L. S. Bartell, J . Chem. fhys., 43, 654 (1965). (a) R. W. Kilb, C. C. Lin, and E. B. Wilson, Jr., J . Chem. fhys., 26, 1695 (1957); (b) L. Pierce and L. C. Krisher, ibid., 31, 875 (1959); (c) K. M. Sinnott, ibid., 34, 851 (1961); (d) L. C. Krisher and E. B. Wilson, Jr., ibid., 31, 882 (1959). S. S. Butcher and E. B. Wilson, Jr., J. Chem. fhys., 40, 1671 (1964). H. N. Volltrauer and R. H. Schwendeman, J . Chem. fhys., 54, 260 (1971). L. S. Bartell, J. P. Guillory, and A. T. Parks, J . fhys. Chem., 69, 3043 (1965). K. P. R. Nair and J. E. Boggs, J . Mol. Struct., 33, 45 (1976). P. L. Lee and R. H. Schwendeman, J. Mol. Spectrosc., 41, 84 (1972). H. N. Volltrauer and R. H. Schwendeman, J . Chem. fhys., 54, 268 (1971). A. de Meijere and W. Luttke, Tetrahedron, 25, 2047 (1969). E. G. Codding and R. H. Schwendeman, J . Mol. Spectrosc., 49, 226 (1974). J. S. Muenter, J . Chem. fhys., 48, 4544 (1968). A. S. Esbiti and E. B. Wilson, Jr., Rev. Sci. Insfrum., 34, 901 (1963). D. R. Lide, Jr., J. Chem. fhys., 33, 1514 (1960). D. R. Lide, Jr., and D. Christensen, J. Chem. fhys., 35, 1374 (1961). J. K. G. Watson, J . Chem. fhys., 48, 4517 (1968). D. R. Lide, Jr., and D. E. Mann, J . Chem. fhys., 27, 868 (1957). D. R. Lide, Jr., and D. E. Mann, J . Chem. fhys., 27, 874 (1957). C. R. Quade and C. C. Lin, J . Chem. fhys., 38, 540 (1963). E. B. Wilson, Jr., and J. B. Howard, J . Chem. Phys., 4, 260 (1936); E. E. Wilson, Jr., J. C. Decius, and P. C. Cross, "Molecular Vibrations", McGraw-Hill, New York, 1955. J. V. Knopp and C. R. Quade, J . Chem. fhys., 53, 1 (1970). J. V. Knopp and C. R. Quade, J . Chem. Phys., 48, 3317 (1968). S. Kondo, E. Hirota, and Y. Morino, J. Mol. Spectrosc.,28, 471 (1968). R. A. Creswell, P. J. Manor, R. A. Assink, and R. H. Schwendeman, J . Mol. Spectrosc., 64, 365 (1977).
N
Microwave Spectrum, Structure, Dipole Moment, and Internal Rotation of cis -Azomet hane James F. Stevens, Jr., R. F. Curl, Jr.,*
and Paul S. Engel"
Chemistry Department, Rice University, Houston, Texas 7700 7 (Received November 10, 1978) Publication costs assisted by the National Science Foundation
The microwave spectra of the ground state of cis-azomethane and of the ground and first excited torsional state of cis-azomethane-d6have been observed and analyzed (for the excited torsional state, Q-branch assignment only). Values for r(C-N) and LCNN of 1.48A and 119.3",respectively, are obtained from the rotational constants of these two isotopic species and an assumed r(N=N) of 1.254 A. Analysis, neglecting possible top-top coupling, of the internal rotation splittings observed in the ground state transitions of the h6 species gives a barrier of 1320 cal/mol, with the uncertainty determined by the inadequacy of the model. This internal rotation analysis also gives an outward tilt of the methyl groups of 6.7 3O. The dipole moment has been determined by measurement of the Stark effect to be 3.27 h 0.1 D.
*
Azodkanes (R-N=N-R) exist as two geometric isomers, of which the trans is by far more common. The usual syntheses of azoalkanes provide exclusively the trans isomer; moreover, this isomer is of lower energy content than cis. The only exceptional compound, difluorodiimide, is more stable in the cis configuration.2 Perfluoroazo0022-3654/79/2083-1432$01 .OO/O
methane was originally thought to exist in the cis form, but more recent work casts considerable doubt upon this concl~sion.~ Both experimental4 and theoretical5 results suggest that, for unhindered azoalkanes, the cis isomer lies about 8 kcal mol-' higher in energy than trans. Although photochemical isomerization of aromatic azo compounds
0 1979 American
Chemical Society
The Journal of Physical Chemistry,
Microwave Spectrum of cis-Azomethane
n
Flgure 1.
Structures of
cis azo
compounds.
has been known since 1937,6it was not until 1964 that this technique was applied to the aliphatic series.' cis-Azomethane, the simpleist azoalkane, and a variety of other cis isomer^^,^ can now be prepared by irradiation of trans. More recently, two chemical synthesesl0?l1of cis-azoalkanes have appeared, The two isomers exhibit large differences in physical and chemical proper tie^;^ for example, cisazomethane boils 93O higher than trans." Because cis acyclic azoalkanes have only recently become available, structural studies of the cis azo group have been confined mainly to cyclic compounds; X-ray crystallographic determinations have been carried out on compounds 1-313 while 4 was studied14 by microwave spectroscopy (Figure 1). Because of the constraints imposed by cyclic structures, these studies provide little information about the geometry of simple acyclic cis-azoalkanes. Although this alone is sufficient reason to determine the structure of an acyclic cis-azoalkane, we were interested in a calculation which indicates that the splitting between the bonding and antibonding combination of n orbitals depends strongly on the C-N=N angle.15 For cis-azoisopropane, this angle was predicted to have the surprisingly large value of 130-140". A similar correlation16 of bond angle with UV absorption maximum which seems valid for acyclic azo,alkanesghas now been extended to cyclic analogues. l' Recent thermocheinical work has demonstrated that in contrast to cycloalkenes,five-membered azoalkanes possess less strain energy than six-membered ones.18 This observation can be explained if lone pair repulsion contributes substantially to the ring strain and if this effect is reduced by constraining the azo linkage to a smaller ring. In the absence of a ring, azoalkanes would presumably exist with an unusually sinal1 bond angle, were it not for repulsion between cis alkyl groups. The determination of the structure of cis-azomethane should aid in the assessment of the relative importance of lone pair repulsion and alkyl-alkyl repulsion. An important addlitional point of interest in the microwave spectrometric study of cis-azomethane lies in the effects of internal rotation of the methyl groups upon the microwave spectrum. On account of the proximity of these methyl groups, it is piossible that the top-top interactions might have a considerable effect on the barrier to internal rotation. By study of' enough torsional states and isotopic species, sufficient information might be obtained to allow analysis of top-top coupling.
Experimental Section trans-Azomethane obtained from Merck Sharp and Dohme had no observable spectrum in the microwave region scanned and was therefore used without purifi-
Vol. 83, No. 11, 1979
1433
cation. Isomerization to cis was accomplished by irradiating trans-azomethane at -196 "C through Pyrex with a 450-W Hanovia lamp for 45 min. Although UV spectroscopy revealed new absorption due to the cis isomer, the exact percentage conversion could not be determined on account of bverlap of cis and trans bands. However, conversion was small, as judged by the negligible change in absorbance of trans. Further irradiation did not appear to increase the percentage of cis, probably because of cis and trans spectral overlap and the use of broad band irradiation. The microwave spectrum of this mixture of cis- and trans-azomethane was recorded using a Hewlett-Packard 8460A MRR spectrometer. Although initial work used a static sample charge, in later observations the sample was allowed to flow through the Stark cell in order to eliminate absorption lines believed to arise from the tautomer of azornethane (formaldehyde methylhydrazone CH2=NNHCH,).l2 The time consuming nature of this preparation relative to the short observation time necessitated a new synthetic approach. 2,3-Dimethyl-7,7-spirocyclopropyl-2,3-diazabicyclo[2.2.l]hept-5-ene (5) was prepared from the Diels-Alder adductlg of 5,5-spirocyclopropylcyclopentadieneZ0and ethylazodicarboxylate by reduction with lithium aluminum hydride.21 Pyrolysis of 5 was effected
v
by allowing it to vaporize a t room temperature through a 0.6-in. x 2 f t length of heated glass tubing filled with glass helices.11~22 The products were led directly into the Stark cell of the microwave spectrometer. It was found that a pyrolysis temperature of 230 "C produced a spectrum 2 to 3 times more intense than the one obtained from the irradiated trans-azomethane sample without introducing extraneous lines. (Extraneous lines were observed when the temperature was raised to 320 "C.) The pressure a t the end of the Stark cell closest to the vacuum pump was kept between 10 and 100 mtorr depending upon whether a high or low resolution scan was desired. No pressure measurement was made a t the end of the Stark cell into which the sample was introduced. cis-Azomethane-d6was prepared by the pyrolysis of 5-d6 in the same manner as the protio species. 5-d6 was synthesized by the reduction of its precursor (2,3-dicarbomethoxy-7,7-spirocyclopropyl-2,3-diazabicyclo [ 2.2.11hep t-5-ene) with LiA1D4 instead of LiA1H4.
Assignment of the Spectrum The ground state spectrum in the 26.5-40-GHz region was assigned for both cis-azomethane and cis-azomethane-ds. In addition, a Q-branch assignment was made for the first excited torsional state of the deuterated species. The normal species assignment was accomplished in the following manner. As can be seen in Figure 2, the fast scan spectrum is dominated by three Q-branch transitions, which were readily assigned with the aid of a spectrum predicted from an assumed structure and some permutations of possible assignments. The three tranwith sitions were found to be of the family J2,J-2 J1,+-l J = 6,7,8. Using the resulting (A-C) and K , the remaining &-branch transitions with sufficient intensity in the 26.5-40-GHz region were soon measured and assigned. In a slow scan, most transitions were found to consist of a symmetrical internal rotation triplet. The same kind
-
1434
The Journal of Physical Chemistry, Vol. 83, No. 1 I, 1979
J. F. Stevens, R. F. Curl, and P. S. Engel
TABLE I: Transition Frequencies of the Ground State of cis-Azomethane
3,,
+
-
A v(cd),
A v( obsd),
A v ( calcd),
MHz
MHz
MHz
3,,
36 279.40a
0.11
-0.57
7.50
6.86
312
27 250.38
-0.14
-0.28
2.43
2.25
39 35 27 30 35 39 31 39 38 33
-0.05 0.00 0.05 -0.02 0.01 -0.01 -0.20 0.46 -0.30 0.03
-0.97 -3.42 -1.66 -3.43 6.48 -2.77 -0.06 -0.49 -1.69 -2.03
7.79 10.72 4.62 6.31 8.77 4.21 1.78
7.41 10.19 4.77 6.73 9.42 4.94 1.73 1.0 1.4 -2.8
42, * d I 4 6,s e 606 624
615
725 '26
g3, 212 404
81? +
+
EE
ObSd calcd, MHz
transition 3,,
A, A, -EE
A,A, v(obsd), MHz
g2, 10,
* 313
5,, +- 4,, 937 844 +
825.60 599.90 044.26 042.95 050.98 116.51 487.10 136.19 949.43 988.95
g g ii
- (EA, t
A u(obsd),
MHz
A1E)/2b A u(calcd),
MHz
5.5" 9.5d e
5.05' 8.8d 4.1" 0.3d
7.71 11.09 4.45 6.36 8.82 4.33 1.90
f f f f f f f f f
g g
h
-9.9" 4.3d
a Estimated uncertainties in frequencies 20.10 MHz. Usually the EE-A,E and the EE-EA, differences are not expe'cted t o be resolved so that the average value is given. " EE-A,E. EE-EA,. e Interfering Stark effect. f When A,E and EA, are not resolved, the predicted EE - (EA, + A,E)/2 is the same as A,A, - EE. N o splitting is resolved, Not observed.
,.....1,
.........................................
T ' . . , . ' .
...
3 ?i5 33 32 2s 28 27 OHz 34
36
SI
SO
Figure 2. Broad scan microwave spectrum of CiS-aZOmethane-h6. 82,6
3-
TABLE 11: Rotational Constants of cis-Azomethane
'i,7
t I 8B MHz
A B
c
A~
x 103
lo3 lo3 io3
AJK x A j x ~ J X 6 K x 103
h6 species A, A,, MHz
d, species ground state
16285.20( 44)" 6739.10(22) 5067.39(20) 5.6 -22.1 11.1 -5.2 5.5
12471.78( 13)a 5344.90(16)a 4 158.98( 14)" -3.3 12.4 11.5 -0.2 -3.7
a The quoted uncertainties are three estimated standard deviations. The centrifugal distortion uncertainties are much larger than the constants. Thus AE(h,) = 5.6 I 223 kHz. Therefore the centrifugal distortion constants should be used only t o reproduce the calculated spectrum.
Figure 3. High resolution microwave scan of 8, 8,, lines of cis-azomethane-h,. The triplet structure arises from internal rotation. The smaller structure is believed to arise from I4N nuclear hyperfine structure. +
of splitting has been found in several previously studied molecules having two equivalent internal rotors and a barrier sufficiently high to cause near degeneracy of the A,E and EA1 symmetry state^.^^-^^ A typical slow scan (Figure 3) shows that some transitions have further, smaller splittings which are probably due to nuclear quadrupole coupling of the two 14Nnuclei. Because these splittings are near the limits of resolution, no further effort was made to determine I4N nuclear quadrupole coupling constants. Some transitions were not observed to be
triplets but subsequent predictions usually indicated that these transitions would have unresolvably small splittings. The strongest R-branch transition in the 26.5-40-GHz region, 21,2 lo,l, was readily assigned since it was the only strong line with the correct Stark effect within 1 GHz of the frequency predicted from an assumed structure. In addition, it was possible to predict from the splittings of the &-branch transitions that the internal rotation splitting of this transition should be 1.6 MHz, which is within experimental error of the observed splitting (1.7 MHz). The additional R-branch transitions listed in Table I were easily predicted and observed. The AIAl transitions, which are expected to approximate a rigid rotor most closely, were fit with a rotational Hamiltonian which included centrifugal distortion. The constants resulting from this fitting are given in Table 11,
-
The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
Microwave Spectrum of cis-Azomethane m I7 w
a0
m a4
as
bL
. 1.. Y
... . ...
ao
n
1435
17 mr
t8
c
9 3
Figure 4. Broad scan microwave spectrum of cis-azomethane-d,. identified by (0,l).
TABLE 111: Transition Frequencies (MHz) of the Ground TorsionalState of cis-Azomethane-d6 obsd - calcd, transitions Vobsd, MHza MHz 0.00 32 995.74 31.3 + 20.2 %,, + 3.1' 3 1 783.66 0.00 32.2 31.3 27 605.03 -0.08 53,z 5z,3 37 153.62 0.07 63,3 + %,4 35 263.10 -0.07 '2,s '1,6 37 067.18 0.10 734 72,s 33 184.79 0.06 32 328.21 -0.09 71,6 + 70,, 8395 8296 31 362.64 -0.04 '3,6 gZ,7 30 273.66 0.05 92,7+ 91,s 30 215.53 0.07 102,s 30 326.60 -0.00 m , 9 122.10 34 931.18 -0.02 -0.01 113.8 11,,9 3 1 818.68 +
+
+
+ -
+
+
a
Estimated uncertainty t 0 . 0 5 MHz.
and v(obsd - calcd) and Av(cd) are shown in Table I. As one might expect from the relatively small number of lines being fitted, the Centrifugal distortion constants are uncertain by an amount much larger than the constants themselves and, therefore, they should be regarded only as a set of parameters which reproduce the spectrum. Since omission of centrifugal distortion substantially increases the standard deviation, inclusion of this effect probably improves the accuracy of the rotational constants. The analysis of the other internal rotation states will be discussed in a subsequent section on internal rotation. The broad scan spectrum of cis-azomethane-d6 (Figure 4) is again dominated by Q branches. The ground and an
Ground state lines are identified by (0,O).Excited torsional state lines are
excited torsional state of cis-azomethane-d6were assigned and fit in the same manner as the normal species except that the initial Q-branch assignment was made for the family J3J-3 J2cl-2,with J = 10, 11,12. The ground state of the deuterated species shows no internal rotation splittings because the substitution of deuterium for hydrogen approximately doubles the top moments of inertia. The observed fPEquencies for the ground state of the d6 species are listed in Table 111,and the results of fitting the observed transitions with rotational and centrifugal distortion constants are shown in Table 11. Again, the estimated uncertainties in the centrifugal distortion constants are much larger than the constants. The observed transition frequencies of the first excited torsional state of cis-azomethane-d, are listed in Table IV. Only Q-branch transitions were assigned because the R-branch transitions expected in this microwave region were predicted to be of low intensity; however, the Qbranch transitions are sufficient for internal rotation analysis. It is interesting to note that two of the high J transitions appeared as quartets instead of triplets due to the splitting of the AE and EA symmetry species. On the other hand, the AE-EA or the larger AA-EE internal rotation splittings were unresolvable in most of the transitions, in accord with prediction. The low intensities of these lines (caused by the smaller Boltzmann factor for the excited state) resulted in degraded resolution in comparison to the ground state lines. A fit of the observed trans5tions with (A-C) and K for each internal rotation state is shown in Table IV. Two low lying excited methyl torsional states are expected for cis-azomethane because the two tops can rotate +-
TABLE 1V: Transition Frequencies (MHz) of the First Excited Torsional State of cis-Azomethane d6 obsd -
63.3
734 83$ 93,6 l03.7
+
62.4
72,s 82,6 +
+
92.7 102.8
113,8+
112,9
123,9
122,lO
+
AV(A,A, EE)
AV(A,A,- A , E ) ~
A V ( A , A --, EA,)^
calcd
obsd
-0.10
0 0 0 0 -2.90 -6.51 -10.76
A,A, v(obsd)
calcd
obsd
calcd
obsd
35 602.69 33 537.37 31 686.72 30 514.96 30 428.80 3 1 730.11 34 611.17
-0.93 -0.69 0.42 1.14 0.72 -0.32 -0.12
0 0 0 0 -1.55 -3.49 -4.95
t 0.09 t 0.19 t 0.11
0 0 0 0 -2.90 -5.59 -9.45
-0.44 -1.51 -3.10 -5.15
+ 0.39 + 0.25 -0.76 -2.72 -5.63 -9.39
calcd -0.07
+ 0.49 +0.32 +0.83 -3.07 -6.39 -10.68
&A, EE AIE EA, 8615.98 8616.39 8616.71 8616.81 K -0.72934 -0.72929 -0.72926 -0.72924 std dev, MHz 0.84 0.75 0.74 0.64 a The symmetry labeling is done on the basis of the very tentative assignment of this state to the ( 0 , l ) state. The choice of assignment between A,E and EA, is completely arbitrary. --
(A
- C), M[Hz
1436
J. F. Stevens, R. F. Curl, and P. S. Engel
The Journal of Physical Chemistry, Vol. 83, No. 7 1, 1979
TABLE V : Nuclear Spin Statistical Weights of the First Excited Torsional States of cis-Azomethane ~
Internal rotation stateb
~
TABLE VI: Planar Moments of Inertia of cis-Azomethanea
~~
h,, amu A 2
statistical weight of rotational symmetry ee-ooc
eo-oec
561 600 676 1584
528 552 576 1584
( l , O ) a State
A24 A,E EA, EE
528 552 576 1584
561 600 576 1584
a The notation used is (v-, vt ) where v- is the mode associated with the two tops turning in opposite directions as viewed down the respective CN bonds. The symmetry designation of the C3"-X C3"*group of ref 26. The parity of K - , K+ ,. Even-even and odd-odd have the same statistical weights and similarly even-odd and odd-even have equal weights.
in the same or in opposite directions. The fact that no other vibrational satellite system of comparable intensity was found for cis-azomethane-d, suggests that the two excited torsional states have quite different energies, implying that the coupling between the motion of the two tops is large. In principle it is possiblez5to determine the symmetry, and thereby the vibrational assignment, of this excited torsional state by comparing the relative intensities of the internal rotation components to those predicted by the spin weights2, for various symmetries. The spin weights of the internal rotation components of the two first excited torsional states of the d6 species are given in Table V. The two first excited torsional states could be distinguished by comparing the intensity of the AA state to that of the EA state for the two symmetry types of rotational levels. The spin weights given in Table V indicate that a 6% change in relative intensity must be discernable but the experimental signal-to-noise ratio did not permit this intensity measurement with such accuracy. The symmetry labels in Table IV are based on the (probably unwarranted) assumption that this excited torsional state is the (0,l) state. The decision whether to label an internal rotation component EA2 instead of AIE was made completely arbitrarily. Relative intensity measurements comparing the first excited state to the ground state of the d6 species show that this state is 120 f 30 cm-l above the ground state. An attempt was made to observe the natural abundance 13Cspecies but no assignment was found primarily because the weak signals approached the sensitivity limit of the spectrometer.
Structure The planar moments of inertia of the ground states of the two isotopic species were calculated from the rotational constants and are given in Table VI. The values of P, obtained, which are very close to the values expected for the moments of inertia of a CH3 or CD, group, establish the planarity of the heavy atom structure. An insufficient number of isotopic species have been studied to determine the structure by isotopic substitution. However, an attempt was made to determine the structure by locating the C atoms by isotopic substitution, assuming a reasonable CH3 group, the methyl tilt, and a modified version of Kraitchman's equations. Then the resulting carbon coordinates were used to fit the planar moments.
obsd calcd obsd calcd 71.8449 71.61 88.2022 88.43 Pb 27.8861 27.91 33.3122 33.29 pc 3.1467 3.16 6.3507 6.32 Conversion factor used 505 376 MHz amu A 2 .
P,
(0,l)' State AI4 AIE EA, EE
d,, amu A
a
An unreasonably long (1.37 A) r(N=N) resulted, probably because of the slight shrinkage in substitution structures27 compared to ro structures. Essentially, r(N=N) is being elongated to take up all the discrepancy in Pa resulting from the substitution shrinkage effect. Since the r(N=N) distance in the cis-azo compounds previously studied all appear to fall between 1.25 and 1.26 A (see Figure l),the r(N=N) distance in cis-azomethane was taken as 1.254 8. The further assumptions that r(C-H) = 1.085 A, LHCH = 109.5', and that the axis of the methyl group makes an angle of 54' with the a axis (vide infra) led, by least-squares fitting of Paand Pbfor the two isotopic species, to r(CN) = 1.480 and LCNN = 119.3'. The calculated Pa,Pb, and P, values are given in Table VI. It is interesting to note that, if the methyl tilt is reduced to zero, the two isotopic species become more consistent with each other without significantly changing the structure.
Internal Rotation The observed internal rotation structure of the h6 ground state was fit according to the first- and second-order perturbation treatment developed by Piercez4with two parameters: A,, the direction cosine between the top axis and the a axis, and s, the reduced barrier parameter. Only the AIA1-EE splittings were used because the AIE, EA1 component of the triplet structure usually observed might be broadened by unresolved splitting of the AIE from the EA, component. Generally, the calculated values of Au(AJ-EE) differ more from the observed Au(AIA1-EE) than do the observed values of Au(EE - (A,E + EA1)/2), so that omission of the AIE, EA, line has a negligible effect on the fitting. Using I , = 3.16 amu A2, the resulting parameters are A, = 0.59 f 0.04 and s = 36.1 f 0.2 where the quoted uncertainties are three standard deviations. The angle between the top axis and the a axis is 54 f 3' which, when combined with the previous structural determination, gives a methyl tilt of 6.7 f 3'. The barrier to internal rotation calculated from s is 1320 cal/mol. The calculated internal rotation splittings are listed in Table I. As has already been noted, internal rotation splittings are also observed in the torsionally excited state of the d6 species, An estimate of the barrier can be made by determining a 4 Wv(2)from the A1A2-EA2splittings and then using the relationshipz8 AE, = -4.5FA Wv(2'/2
(1)
to determine a tunneling energy splitting between these two states. The tunneling splitting was calculated by diagonalization from the free rotor basis using 17 m values for each top. With the potential interaction terms between the two tops set equal to zero, a barrier of about 1450 cal/mol reproduced the tunneling splitting. In contrast, a barrier of about 1650 cal/mol was needed to reproduce the energy difference of 120 cm-I between the first excited and ground state obtained from relative intensity measurements. Because the assignment of the torsionally excited state is uncertain, the barrier height obtained from
The
Microwave Spectrum of cis-Azomethane
Journal of Physical Chemistry, Vol. 83, No.
11, 1979
1437
Yh AY I
I1
III
m
Figure 5. Possible methyl group conformations of cis-azomethane. The direction of viewing is from the carbon atoms toward the nitrogen atoms in the plane of the heavy atoms. Structure I is referred to as the eclipsed-eclipsed form in the text. Structure I1 is referred to as the eclipsed-staggered form, structure I11 is referred to as skew-skew, and structure I V is referred to as the staggered-staggered form. It is believed that I V corresponds to the actual energy minimum.
the internal rotation splittings of the h6 ground state is likely to be a much more reliable estimate of the barrier.
Dipole Moment In order to determine the dipole moment of cis-azomethane, a measurement of the Stark effects of the 21,2-5,1 of the normal species and 31,3-20,2of the d6 species transitions was made. The Stark cell was calibrated by measuring the Stark effect of the 2,-10 transition of propyne, for which the dipole moment has been determined to be 0.7840 .D.29 A least-squares fit to the frequency measurements of the IMI = 0, 1 components of this transition a t five different voltages yielded a value of 3.20 f 0.14 D for the dipole moment of the normal species. A similar measurement of the IMI = 0, 1, 2 components of the d6 species transition gave a dipole moment of 3.33 f 0.14 D. The uncertainties shown are three standard deviations calculated on the assumption that only the frequency measurements of propyne, cis-azomethane, and cis-azomethane-de were liable to error and ignoring any error in voltage readings. The best estimate of the dipole moment is taken to be the average of the results for the two isotopic species, 3.27 f 0.1 D. Discussion The most striking; aspect of the structure of cis-azomethane is the enormous crowding of the two methyl groups. In cis-2-butene it has been established30 that the protons of the methyl groups eclipse the double bond as is the normal situation for methyl groups attached to double bonds. The observed cis hydrogen-hydrogen distance is 1.93 A, which is smaller than twice the van der Waals radius (2.4 The resulting bumping is the probable reason why the barrier to internal rotation in cis-2-butene (750 is substantially less than the barrier in propylene (1950 ~ a l ) The . ~ ~cis hydrogen-hydrogen distance of the eclipsed-eclipsed form of cis-azomethane calculated on the basis of the heavy atom structure determined here is 1.47 A, which is 0.9 A smaller than the sum of the van der Waals radii. It is therefore very unlikely that the eclipsed-eclipsed form will be the energy minimum. Figure 5 depicts some of the possibilities for the energy minimum for internal rotation. The steric strain should be considerably relieved in either the eclipsed-staggered form (TI), the skew--skew form (III), or the staggeredstaggered form (IV). The unsymmetrical forms I1 and I11 would appear to relieve the steric strain without raising the internal rotation energy as much as the staggeredstaggered (IV). However, the number of equivalent energy minima is increased from nine to eighteen with either of these unsymmetrical structures as the energy minimum
Figure 6. The structure of cis-azomethane which resuks from this study.
conformation. (The nuclear permutation inversion symmetry26 remains the same.) As with any inversion problem, the introduction of additional minima by raising a barrier a t the more symmetrical configuration (Figure 5 , I) can cause the levels of the first excited torsional state to collapse to the ground state and become a tunneling splitting. The extent of such a collapse depends on the difficulty in tunneling between the new minima. Obviously, if the potential bump a t the eclipsed-eclipsed configuration (Figure 5, I) were very small, it would be difficult to detect because it would merely cause an anomalously low lying excited torsional state. In the other limit of complete gearing of the two tops with minima a t the eclipsed staggered form (Figure 5 , 11),there are three sets of six equivalent minima with very small tunneling rates between the different sets. The tunneling caused by internal rotation within a single set would be analogous to that of a sixfold rotor and give rise to four torsional sublevels. Since the spectrum shows triplet internal rotation structure, strong gearing appears to be ruled out. Moreover, the apparent absence of low energy excited torsional states suggests that neither of the unsymmetrical forms (Figure 5 , I I and 111) is the minimum energy conformation. Thus by a process of elimination we conclude that the methyl conformation must be the staggered-staggered form (Figure 5, IV). With this choice of conformation, the overall structure proposed here is shown in Figure 6. This conclusion concerning the methyl conformation is in agreement with an ab initio c a l ~ u l a t i o nwhich , ~ ~ found the symmetrical staggered-staggered (Figure 5 , IV) conformation to be of lowest energy. A recent infrared of cis-azomethane supports CZusymmetry but does not distinguish between I and IV (Figure 5 ) . In the geometry-optimized calculations of Flood, Pulay, and B ~ g g sthe , ~ ~energies of configurations I, 11, and I11 relative to IV were 1.13, 0.41, and 0.11 kcal/mol, respectively. The resulting34barrier to internal rotation appears to be about half of that obtained from the observations reported here. The tilt angle calculated34for the staggered-staggered configuration (IV) is 3.5", which is about half of the 6.7' value obtained here. These trends in tilt and barrier might be an indication that the ab initio calculation is in some way underestimating the crowding. The geometry optimized structure34with r(N=N) = 1.224 A, r(CN) = 1.499 A, and LCNN = 121.6' does crowd the methyl groups less than the structure reported here. It was noted in the introduction that an unusually large LCNN ( 130-140') was predicted15 for cis-azoisopropane. Although the structure found here does not show such a large LCNN, the fact that cis-azoisopropane is even more sterically crowded than cis-azomethane may cause opening of LCNN. Resolution of this question will require structural study of cis-azoisopropane. Comparison of LCNN in azomethane (119.3') with LC-C=C in cis-2butene (126.7°)30reveals a difference which is clearly a
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The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
result of lone pair repulsion. This effect has been invoked to explain ring strainla and UV spectral6 of azoalkanes. Finally, it is worth noting that it should be possible to prepare cis-azomethane-dl and experimentally establish the methyl group orientation from the rotational spectrum, thereby finally settling the conformational question.
Acknowledgment. We thank Norman C. Craig for bringing the work of Hayden, Tuazon, and Fateley to our attention. References and Notes (1) Supported by National Science Foundation Grants CHE 76-22367 and CHE 76-20032 and Robert A. Welch Foundation Grants C-071 and C-499. (2) G. T. Armstrong and S. Marantz, J. Chem. Phys., 38, 169 (1962). (3) The electron diffraction work leading to the cis structure can be found in C. H. Chang, R. F. Porter, and S. H. Bauer, J . Am. Chem. Soc., 92, 5313 (1970), while the IR and Raman work which supports the trans structure is in R. A. Hayden, E. C. Tuazon, and W. G. Fateley, J . Mol. Struct., 16, 35 (1973). (4) P. S. Engel and D. J. Bishop, J . Am. Chem. Soc., 97, 6754 (1975). (5) Some recent theoretical work on azoalkanes can be found in R. N. Camp, I. R. Epstein, and C. Steel, J . Am. Chem. Soc., 99, 2453 (1977); J. M. Howell and L. J. Kirschenbaum, ibid., 98, 877 (1976); E. R. Taiaty, A. K. Schwartz, and G. Simons, ibid., 97, 972 (1975). (6) G. S. Hartley, Nature (London), 140, 281 (1937). (7) R. F. Hutton and C. Steel, J . Am. Chem. Sac., 86, 745 (1964). (8) L. D. Fogel and C. Steel, J . Am. Chem. Soc., 98, 4859 (1976). (9) P. S. Engel, R. A. Meiaugh, M. A. Page, S. Szilagyi, and J. W. Timberlake, J . Am. Chem. SOC.,98, 1971 (1976). (10) P. S. Engel, Tetrahedron Left., 2301 (1974). (11) S. F. Nelsen, J . Am. Chem. Soc., 96, 5669 (1974). (12) M. N. Ackermann, N. C. Craig, R. R. Isberg, D. M. Lauter, R. A. MacPhail, and W. G. Young, J . Am. Chem. Soc., 99, 1661 (1977).
J. D. Swalen (13) T. Gtterson, C. H. R. Romming, and J. P. Snyder, Acta Chem. Scand., Sect. 6 ,30, 407 (1976). (14) R. Suenram, J . Mol. Struct., 33, 1 (1976). (15) K. N. Houk, Y. M. Chang, and P S. Engel, J . Am. Chem. Soc., 97, 1824 (1975). (16) N. C. Baird, P. DeMayo, J. R. Swenson, and M. C. Usselman, Chem. Commun 314 (1973). (17) N. C. Baird, Can. J . Chem., 57, 98 (1979). (18) P. S. Engel, J . Am. Chem. Soc., 98, 1972 (1976). (19) W. R. Roth and K. Enderer, Justus Liebig's Ann. Chem., 730, 82 (1969). (20) B. F. Hallam and P. L. Pauson, J . Chem. Soc., 646 (1958). (21) J. E. Anderson and J. M. Lehn, J . Am. Chem. Soc., 89, 81 (1967). (22) The authors thank Professor S.F. Nelson for suggesting 5 as a better precursor to cis-azomethane than the published" compounds. (23) P. Kasai and R. J. Myers, J . Chem. Phys., 30, 1069 (1959). (24) L. Pierce, J . Chem. Phys., 34, 498 (1960). (25) E. Hirota, C. Matsummura, and Y. Morino, Bull. Chem. SOC.Jpn., 40, 1124 (1967). (26) R. J. Myers and E. B. Wilson, Jr., J . Chem. Phys., 33, 186 (1960). (27) W. Gordy and R. L. Cook, "Microwave Molecular Spectra", Interscience, New York, N.Y., 1970, p 534. (28) D. Herschbach, J . Chem. Phys., 31, 91 (1959). (29) J. S. Muenter and V. W. Laurie, J . Chem. Phys., 45, 855 (1966). The value of 0.7840 D used here is obtained from the value of 0.7809 D reported by Muenter and Laurie by using the new standard value of the dipole moment of OCS reported by Muenter, J . Chem. Phys., 48, 4544 (1968). (30) S . Kondo, Y. Sakurai, E. Hirota, and Y. Morino, J . Mol. Spectrosc., 34, 231 (1970). (31) L Pauling, "The Nature of the Chemical Bond", Cornell University Press, Ithaca, N.Y., 1960, p 260. (32) T. N. Sarachman, J . Chem. Phys., 49, 3146 (1968). (33) D. R. L i e and D. E. Mann, J . Chem. Phys., 27, 868 (1957); E. Hirota, ibid., 45, 1984 (1966). (34) E. Flood, P. Pulay, and J. E. Boggs, J . Ma/. Struct., 50, 355 (1978). (35) M. N. Ackermann, N. C. Craig, R. R. Isberg, D. M. Lauter, and E. F. Tacy, J . Phys. Chem., in press.
Optical Wave Spectroscopy of Molecules at Surfaces J. D. Swalen' IBM Research Laboratory, San Jose, California 95 193 (Received November 27, 1978)
Publication costs assisted by the IBM Research Laboratory
Optical spectra of molecules at surfaces have been measured with high sensitivity by new techniques utilizing optical guided and surface waves. The similarities and differences between these methods and microwave spectroscopy will be outlined, as well as a brief comparison with some of the older conventional optical techniques. Both polarization and spatial information of absorbing molecules can be determined by specific experimental arrangements and by computer analyses of the complex wave equation derived from Maxwell's equations. Results are given on the absorption of a monolayer of a cyanine dye on the surface of an optical waveguide and on a silver surface which can support plasmon surface waves. Through the use of these spectroscopic techniques we have obtained new information about molecular monolayers on surfaces and these results will be presented and discussed.
Introduction It is a pleasure to contribute to this "Festschrift" in honor of E. Bright Wilson, Jr., and describe some of our work on optical wave spectroscopy. A related area, microwave spectroscopy, has been a field of research of Wilson and his students for a number of years to elucidate molecular s t r u ~ t u r e s l -and ~ potential energy surfaces, particularly for molecules with internal rotation.44 Many concepts and ideas learned there have been carried over into NMR, ESR, and laser spectroscopy of gases, and now laser spectroscopy of surface molecule^.^^ The similarities, as well as the differences of optical wave spectroscopy and microwave spectroscopy, show how one area of research can spawn another. In this paper, our methods and some recent experimental results from optical wave spectroscopy will be presented. 0022-3654/79/2083-1438$01 .OO/O
Before we start to discuss and describe the optical wave spectroscopic technique in detail, it may be useful to give a few general comparisons between microwave and optical wave spectroscopy. One obvious difference between the two is a scaling of wavelength. Microwaves have wavelengths of a few millimeters to a few centimeters while optical waves in the visible portion of the electromagnetic spectrum have wavelengths of a fraction of a micrometer. Maxwell's equations, however, apply in both cases and will be used to derive a number of useful results and comparisons. A further comparison can be made regarding the skin depth, i.e., the depth to which the radiation penetrates the conducting surface of the guide. In the microwave region, this depth is about 100 nmloJ1 while in the optical region, it is only of the order of 1 nrn,l2 the skin depth being
0 1979 American
Chemical Society
