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Reinvestigation of the microwave spectrum of cyanocyclobutane: Assignment of the axial conformer
JOURNAL OF MOLECULAR SPECTROSCOPY 123,469-475 (1987)
Reinvestigation of the Microwave Spectrum of Cyanocyclobutane: Assignment of the Axial Conformer WALTHER CAMINATI AND BIAGIO VELINO Istitutodi Chimica Fisica e Spettroscopiadell’lJniversitd.Viale Risorgimento4, 40126 Bologna, and Istitutodi SpettroscopiaMoleculare de1 CNR, Via de’CastagnoliI, 40136 Bologna, Italy
MARWAN DAKKOURI Departmentof Chemistry, Universityof Ulm, Oberer Eselsberg, 7900 Ulm, West Germany
AND LOTHAR SCHAFER, KHAMIS SIAM, AND JOHN D. EWBANK Departmentof Chemistry and Biochemistry, Universityof Arkansas, Fayetteville,Arkansas 72701
The microwavespectrumof the axial conformerof cyanocyclobutanehas been assignedon the basis of its ab initio structure.From dipole moment and relativeintensity measurementsit has been possibleto determinethe relativeenergywith respectto the previouslyassignedequatorial conformer:E (axial) - E (equatorial)= 258 & 50 cm-‘. 0 1987Academicpress, ~nc. INTRODUCTION
In two earlier microwave investigations of cyanocyclobutane (CYB; Fig. 1) the rotational spectra of the equatorial conformer and of some vibrational excited states were assigned (1, 2). At that time, relying on an infrared investigation (3) of the compound, it was believed that the potential function for the ring-puckering mode contained only one minimum. In a recent electron dilhaction and ab initio study of dicyanocyclobutane it was found (4) that the local molecular geometries for the axial and equatorial cyano groups at the cyclobutane ring are significantly different. Thus, the possibility had to be considered that the previous search for the axial conformer of CYB in its microwave spectrum with standard structural parameters was misleading, and that the axial form had been overlooked in the earlier work. To obtain information concerning this question, we decided to perform ab initio geometry refinements for axial and equatorial CYB and to use the calculated geometry of the axial form as a basis to search for its microwave spectrum. In several of our recent investigations (5-8), we have found such combined techniques to be powerful methods of conformational analysis. In the cases of 2-methoxyethylamine (6), methylallyl alcohol (7), and glycine (8), specifically, the calculated geometries were crucial in the detection of a second conformer which originally was overlooked or could not be assigned. As the results presented below will show, for CYB our ab initio calculations afforded a relatively accurate prediction of the microwave spectrum of the previously unobserved axial conformer whose spectroscopic detection was then straightforward. 469
0022-2852187 $3.00 Copyright0 1987 by Academic F?Bs, Inc. Au right.3of reproductionin any form reserved.
470
CAMINATI ET AL. H8
‘Cz'
;;y>( H12'
H9
>cq c4
-qq7
'H13
FIG. 1. Atom numbering for cyanocyclobutane. COMPUTATIONAL
PROCEDURES
Geometry refinements for this study were performed using Pulay’s gradient method (9), his program (IO), and the 4-2 1G basis set (II) augmented with d-functions on TABLE I 4-2 lG* ab Initio Optimized Structural Parameters for the Axial and Equatorial Conformations of Cyanocyclobutane AXI AL Bond Distances 2 1 ; 3 a 9 10 11 7
(A)
: 2 2 2 3 3 6
Bond Angles 4 1 5 6 : 6 i 3 2 a a 22 9 2 9 2
(Des) 2 2 2 5 1 1 3
:
2 3
a 2
:: 11 7
: 3 6
2 10 1
3 3 a
2 2 2 2
1 1 1 1
: 9 9 7 7 4 4 4 10 10 10 11 11 11
; 2 2 6 6 3 3 3 3 3 3 3
: 1
a
1.5595 1.0839 1.4681 1.5493 1.0830 1.0847 1.0848 1.0839 1.1306
:
: 1 2 2 2 2 2 2 2 2 2
88.13 116.84 112.61 108.72 88.88 116.93 117.93 110.08 112.00 109.61 88.86 111.48 117.21 109.34 179.70
s
6
-17.2 -136.5 96.6 -138.2 102.5 -24.4 95.9 -23.4 -150.3 -48.9 180.0 17.3 137.4 -94.0 -95.4
a
9
i
a 9
24.7 153.3 137.5 -102.3 26.3
EQUATORIAL 1.5555 1.0846 1.4658 1.5498 1.0835 1.0842 1.0853 1.0836 1.1305 88.36 111.09 118.03 108.95 88.15 116.65 118.21 110.99 111.72 109.64 88.77 iii.58 117.18 109.29 179.21 -19.2 92.8 -140.3 -140.1 -28.0 98.8 93.4 -154.6 -27.7 52.1 180.0 19.3 138.7 -92.6 -93.5 26.0 154.6 139.4 -101.1
27.5
471
AXIAL CYANOCYCLOBUTANE
carbon and nitrogen following the procedure described in Ref. (I 0). Previous experience (12,23), based on a large number of studies, supports the expectation that the procedure applied will engender molecular structures for CYB which are sufficiently accurate for the current study. All geometries were optimized until the largest residual forces in the final cartesian coordinates were
DETAILS
CYB was prepared in accordance with the procedure described in Ref. (4). The spectrum was measured with a computer-controlled spectrometer in the frequency range 26-40 GHz with the cell thermostat at approximately -30°C. ASSIGNMENT OF THE ROTATIONAL
SPECTRUM
The rotational constants of both conformers were calculated from the ab initio structure. Then the calculated constants of the axial conformer were corrected with the differences found between the ab initio and experimental rotational constants of the previously assigned equatorial form.
TABLE II List of Measured Line Frequencies for the Axial Conformer of Cyanocyclobutane (MHz) J'(Ka.K,)
+
J”W,&)
4C1.3) - 3(0,3) 6(0.6) - 5(0,5) 6(1;5) - 5(1,4) 6(2,4) - 5(2,3) 6(3,31 - 5(3,2 1 6(5) - 5(5)a 7i1;71 - 6(1,6) 7t2.6) - 6t2.5) 7(3;5) - 6(3,4 7(4) - 6(4)a 1 7(6) - 6(6)a) 9(0.9) - 8C1.7) loi4;6j -1oi3;aj 11(4,7) -11(3,91 12t4.8) -12(3,10) 13(4,9) -13(3.11) 14(4,10)-14C3.12) 15(4,12)-15t3.12) 16(3.13)-16C2.15) 16(4;12)-16(3;14) 17(4,141-17(3,14) 18(4,15)-lE(3.15) 19(4,16)-19(3.16) 20(4.17)-2OC3.17) 21(4,18)-21t3.18) 22(4,19)-22t3.191 23(4,19)-23(3,21) a) Asymmetry
degenerate
J'(K,,K,)
26640.49 30967.11 31615.22 31264.84 31154.46 31139.31 35640.99 36277.84 36348.00 36337.52 36328.47 37319.42 34909.79 34897.61 34892.91 34902.31 34933.91 33622.22 30031.06 35108.16 32528.31 31789.38 30911.73 29893.67 28738.73 27455.31 38793.04 K-1
doublets.
+
J (Ka.Kcl II
5C1.4) - 4(0,4) 6t1.6) - 5(1,5) 612.5) - 5t2.4) 6C3.4) 5(3,3 1 6(41 - 5(4)a 7tO.7) - 6t1.5) 7(1,6) - 6(1,5) 7C2.5) - 6(2,4) 7(3,4) - 6t3.3 - 6(5)a i 7(5) 8CO.8) - 7(1,6) lO(4.7) -1OC3.7) ll(4.8) -11(3,8) 12C4.9) -1213.91 13(4;10)-13(3;10) 14(4,11)-14(3,11) 15(3,12)-15(2,14) 15(4,11)-15(3,13) 16(4,13)-16(3,13) 17(3,14)-17(2,16) 18(3.15)-18(2,17) 19(3,16)-19(2,18) 20(3.17)-20/2,19) 20(4,16)-20(3,18) 21(4,17)-21(3.19) 22(4,18)-22C3.20)
32293.18 30563.02 31105.81 31150.37 31142.87 28968.89 36864.14 36528.00 36357.69 36331.19 33239.18 34777.01 34666.14 34509.32 34293.05 34002.92 28848.84 34998.25 33135.69 31460.28 33158.45 35141.70 37419.71 36354.51 36979.05 37782.31
472
CAMINATI ET AL.
The first assigned transitions were the 73 + 63 and 63 + 53 Km1asymmetry doublets, which were measured with the radiofrequency microwave double resonance technique (14), pumping at about 6 MHz. They were found within a few tens of a MHz with respect to the calculated values. Other paR-type transitions were then measured with the conventional Stark modulation technique. Next it was possible to assign p&branch transitions, starting with 16(4, 12) + 16(3, 14) and 16(4, 13) + 16(3, 13), as they were radiofrequency pumped through the 16(4) K-i doublet at about 129 MHz. Several other pCtransitions were measured. According to the quadrupole coupling constants calculated by translating the values of the equatorial conformer (2) to the inertial axis system of the axial conformer, no quadrupole hypertme structure was observed, within our experimental resolution, for the measured transitions. All the measured transitions are listed in Table II. They have been treated with the Watson quartic reduced Hamiltonian (15), and the obtained rotational and centrifugal distortion constants are shown in Table III. Since the centrifugal distortion constants for the equatorial conformer were not determined in previous studies (I, 2), we extended the analysis of the microwave spectrum to this form. Table IV lists the frequencies obtained, and the corresponding spectroscopic constants are reported in Table III. The transitions 21,1+ lo,, and 22,0 +- 1r.0 reported in Ref. (I) deviated about 1 MHz when inserted in the fit, so they have been excluded. The latter, which falls in our frequency range, was overlapped by a much stronger transition. DIPOLE MOMENT
The displacements of the Stark lobes were measured for the 61,6 + 51,s and 71,6f 6 1,5transitions. The Stark coefficients (16) and the obtained dipole moment components TABLE III Spectroscopic Constants of Axial and Equatorial Conformers of CYB Axial
Equatorial
A/MHz
7591.86(6)
9761.88(3)
B/MHZ
2681.987(7)
2383.396(41
C/MHz
2505.943(8)
2105.118(3)
AJ/kHz
1.33(81 -3.31(4)
AJK/kHz
0.35(l) 0.96(13) 9.7(21)
AK/kHz
8.7(24)
bJlkHz
-0.126(Z)
0.016(21
dK/kHz
0.91(18)
1.6(2)
st. dev./MHz
0.12
"a' J max E a)
Number
60
23
19
258(50)
CO"f.fCm-'
of transitions
in the
fit.
0.15
53
0
TABLE IV List of Measured Line Frequencies for the Equatorial Conformer of Cyanocyclobutane (MHz) J’(K,,K,)
+ J”(K,,K,)
J*(K,.K,)
3lO.3) - 2(0,2) 3(1,21 - 2t1.11 3t2.1) - 2t2.0) 4(1,4) - 3t1.31 4(2,3) - 4(1,3) 5LO.51 - 4(0.4) 5i1;4j - 4ii;3j 611.5) - 511.41 6t2.4) - 5(2;3) 7f1.7) - 6t1.6) 7(2,6) - 7(1,6) 7t3.5) - 6t3.4) 7t3.4) - 6C3.3) 7(4;3) - 6i4;2jc) 7(6,1) - 6(6,O)C) 8C1.8) - 7t1.71 8C2.6) - 7i2.5) 8C3.5) - 7C3.4) 9(0,9) - 8(0,8) 9C2.8) - 9C1.8) lO(2.9) -10(1,9) 11(2.10)-ll(l,lO) ll(3.8) -11(2,10) 12(2,11)-12(1,11) 13(0,13)-1211.11) 14fO.14)-13f1.12) 16(2;15j-l5i3;13j 17(0.171-16(1,15) 17(3,14)-16(4,12) 19(0.19)-18t1.17) a) From ref. [23.
13434.64a) 13878.02a) 13496.35a) 17382.31a) 21175.88a) 222e9.72a) 23102.9ea) 27699.d’) 27159.99b) 30351.86b) 1e770.45a) 31475.29 31491.69 31457.18 31440.41 34656.09b) 36416.64b)
+
J”(K,,K,)
3(1,3) - 2(1,2) 3(2,21 - 2(2,1) 4(0,4) - 3CO.3) 4(1,3) - 3(1,21 4(2,2) - 3(2,1) 5(1,5) - 4(1,4) 5(2,3) - 4(2,2) 6(2,5) - 6(1,5) 7(0,7) - 6(0,6) 7(1,6) - 6(1,5) 7(2,5) - 6(2,4) 7(3,5) - 7(2,5) 7(3,4) - 7(2,6) 7(5,3) - 6(5,2)c) 8(0,8) - 7(0,7) 8(1,7) - 7(1,6) 8C3.6) - 7(3,51 8(3,5) - 8(2,7) 9(1,9) - 8C1.8) lO(O,lO)9(1,8) lO(3.7) -10(2,9) 11(3,9) -11(2,9) 12(0.12)-ll(l,lO) 12(3.10)-12C2.10) 13(3.11)-1312,li) X(3,13)-X(2,13) 16(3,14)-16(2,14) 17(2.16)-16t3.14) 18(0;18)-17(1;16) 19(3,16)-18C4.14)
$,$;;b) 16598:79a) 15376.36a) 14085.158) 39372.99 12746.65a) 33695.47 34985.73 29218.89 37884.38 27988.30 39464.83 b) From ref. Ill.
c, K-1
asymmetry
13043.26a) 13465.75a) 17e77.07a) 18494.64a) 18024.87a) 21714.36a) 22577.78b) 19694.26a)
3loo4.82b) 32283.09b) 31772.94b) 36899.04 37873.10 31446.10 353o3.35b) 36848.74b) 35983.87 38076.95 38949.65b) 28293.71 38796.56 34155.82 32176.42 33018.75 31702.53 28589.03 26828.45 32518.47 38682.97 39413.17 doublets
TABLE V Stark Coefficients and Dipole Moments of Axial CYB J'(Ka,Kc)
613
7l,6
+
J”(KasKc)
IMI
Av/E*
(Hz V-*
cm*,
exp.
talc.
1
-0.047
-0.048
2
0.154
0.159
3
0.507
0.504
4
0.999
0.988
+ 51.5
+ 61 . 5
1,2a)
'a
=
3.81(3)
-0.110
0
wb = 0.0 D (by symmetry) 1.34(3)
0
+* = 4.04(4)
0
PC
a) (MI = 1
=
and IMI = 2 component lines overlapped.
473
-0.107
474
CAMINATI ET AL.
for axial CYB are shown in Table V. The cell was calibrated at -30°C using OCS as standard. CONFORMATIONAL
EQUILIBRIUM
In Table VI the experimental rotational constants are compared with the values calculated from the ab initio structure of axial CYB. It is seen from this table that the new experimental constants are consistent with those calculated for the axial form. Line intensity measurements (17) were carried out on similar lines for the two conformers. The energy difference found between the two ground states is AE0,0 = E (axial)o,o - E (equatorial)O,O = 258 & 50 cm-‘. It is interesting to note that the differences between the rotational constants of axial and equatorial CYB, A, - A,, B, - B,, and C, - C,, are very similar for the experimental and the calculated constants obtained from the ab initio structures. These data are also included in Table VI. CONCLUSIONS
The material presented above allows the conclusion that we have succeeded in observing the microwave spectrum of a previously undetected conformer of monocyanocyclobutane and that the newly discovered transitions are consistent with the axial form of this compound. This finding is somewhat different from the previous microwave (I, 2) and infrared investigations (3) of this system. In line with our recent investigations of methylhydrazino carboxylate (5), 2-methoxy ethylamine (6), methylallyl alcohol (7), and glycine (8), this result emphasizes again the usefulness of highquality ab initio geometries in assigning weak rotational spectra in conformational mixtures. A particular advantage of the calculations is that they predict accurate dif-
TABLE VI Experimental and Calculated Rotational Constants and Shifts of Rotational Constants between the Two Observed Conformers of CYB (MHz)
A Axial
Equat.
ab initio
7591.86
7716.81
8
2681.987
2680.187
C
2505.943
2491.542
A
9761.88
B
2363.396
2373.945
C
2105.118
2102.39
AAa)
“AA=AAx
Exptl.
-2170.02
10282.69
-2565.88
AB
298.591
306.242
AC
400.825
389.152
. -A Eq. , etc.
AXIAL CYANGCYCLOBUTANE
475
ferences in local geometries (12, 13), i.e., conformationally dependent trends in bond distances and angles. For CYFJ, especially the axial form, we plan to obtain more extensive information on the ring-puckering potential surface, taking advantage of the results of the current investigation and exploring in greater detail the vibrational satellites associated with the ring motion. RECEIVED:
November
20, 1986 REFERENCES
1. 3. R. DURIG, L. A. CARREIRA,AND W. J. LAFFERTY,J. Mol. Spectrosc.46, 187-193 (1973). 2. M. Y. FONG AND M. D. HARMONY,J. Chem. Phys. S8,4260-4264 (1973). 3. C. S. BLACKWELL,L. A. CARREIRA,J. R. DURIG, J. M. KARRIKER,AND R. C. LORD, J. Chem. Phys. 56, 1706-1711 (1972). 4. M. DAKKOURI,H. EPHARDT,K. SIAM,L. SCHAFER,AND C. VAN ALSENOY,J. Mol. Struct., in press. 5. W. CAMINATI,A. C. FANTONI, L. SCHAFER,K. SIAM, AND C. VAN ALSENOY,J. Amer. Chem. Sot. 108,4364-4367 (1986). 6. W. CAMINATI,K. SIAM,J. D. EWBANK,AND L. SCI-LXFER, J. Mol. Struct., in press. 7. W. CAMINATI,K. SIAM, J. D. EWBANK, C. VAN ALSENOY,AND L. SCHAFER,J. Mol. Spectrosc.. in press. 8. L. SCHAFER,H. L. SELLERS,F. J. LOVAS,AND R. D. SLJENRAM, .I Amer. Chem. Sot. 102,6566-6568 (1980). 9. P. PIJLAY,Mol. Phys. 17, 197-204 (1969). 10. P. PULAY, Theor. Chim. Acta f&,299-312 (1979). II. P. PIJLAY,G. FOGARASI,F. PANG, AND J. E. Boccs, J. Amer. Chem. See. 101,2550-2560 (1979). 12. L. SCH@ER, J. Mol. Strut. 100,51-73 (1983). 13. L. SCHAEFER, J. D. EWBANK,V. J. KLIMKOWSKI,AND K. SIAM,J. Mol. Struct. 135, 141-158 (1986). 14. F. J. WODARCZVKAND E. B. WILSON,J. Mol. Spectrosc. 37,445-463 (1971). 15. J. K. G. WATSON,J. Chem. Phys. 46, 1935-1949 (1967). 16. S. GOLDEN AND E. B. WILSON, JR.,J. Chem. Phys. 16,669-685 (1948). 17. A. S. ESBITTAND E. B. WILSON,JR.,Rev. Sci. Instrum. 34,901-907 (1963).
Reinvestigation of the Microwave Spectrum of Cyanocyclobutane: Assignment of the Axial Conformer WALTHER CAMINATI AND BIAGIO VELINO Istitutodi Chimica Fisica e Spettroscopiadell’lJniversitd.Viale Risorgimento4, 40126 Bologna, and Istitutodi SpettroscopiaMoleculare de1 CNR, Via de’CastagnoliI, 40136 Bologna, Italy
MARWAN DAKKOURI Departmentof Chemistry, Universityof Ulm, Oberer Eselsberg, 7900 Ulm, West Germany
AND LOTHAR SCHAFER, KHAMIS SIAM, AND JOHN D. EWBANK Departmentof Chemistry and Biochemistry, Universityof Arkansas, Fayetteville,Arkansas 72701
The microwavespectrumof the axial conformerof cyanocyclobutanehas been assignedon the basis of its ab initio structure.From dipole moment and relativeintensity measurementsit has been possibleto determinethe relativeenergywith respectto the previouslyassignedequatorial conformer:E (axial) - E (equatorial)= 258 & 50 cm-‘. 0 1987Academicpress, ~nc. INTRODUCTION
In two earlier microwave investigations of cyanocyclobutane (CYB; Fig. 1) the rotational spectra of the equatorial conformer and of some vibrational excited states were assigned (1, 2). At that time, relying on an infrared investigation (3) of the compound, it was believed that the potential function for the ring-puckering mode contained only one minimum. In a recent electron dilhaction and ab initio study of dicyanocyclobutane it was found (4) that the local molecular geometries for the axial and equatorial cyano groups at the cyclobutane ring are significantly different. Thus, the possibility had to be considered that the previous search for the axial conformer of CYB in its microwave spectrum with standard structural parameters was misleading, and that the axial form had been overlooked in the earlier work. To obtain information concerning this question, we decided to perform ab initio geometry refinements for axial and equatorial CYB and to use the calculated geometry of the axial form as a basis to search for its microwave spectrum. In several of our recent investigations (5-8), we have found such combined techniques to be powerful methods of conformational analysis. In the cases of 2-methoxyethylamine (6), methylallyl alcohol (7), and glycine (8), specifically, the calculated geometries were crucial in the detection of a second conformer which originally was overlooked or could not be assigned. As the results presented below will show, for CYB our ab initio calculations afforded a relatively accurate prediction of the microwave spectrum of the previously unobserved axial conformer whose spectroscopic detection was then straightforward. 469
0022-2852187 $3.00 Copyright0 1987 by Academic F?Bs, Inc. Au right.3of reproductionin any form reserved.
470
CAMINATI ET AL. H8
‘Cz'
;;y>( H12'
H9
>cq c4
-qq7
'H13
FIG. 1. Atom numbering for cyanocyclobutane. COMPUTATIONAL
PROCEDURES
Geometry refinements for this study were performed using Pulay’s gradient method (9), his program (IO), and the 4-2 1G basis set (II) augmented with d-functions on TABLE I 4-2 lG* ab Initio Optimized Structural Parameters for the Axial and Equatorial Conformations of Cyanocyclobutane AXI AL Bond Distances 2 1 ; 3 a 9 10 11 7
(A)
: 2 2 2 3 3 6
Bond Angles 4 1 5 6 : 6 i 3 2 a a 22 9 2 9 2
(Des) 2 2 2 5 1 1 3
:
2 3
a 2
:: 11 7
: 3 6
2 10 1
3 3 a
2 2 2 2
1 1 1 1
: 9 9 7 7 4 4 4 10 10 10 11 11 11
; 2 2 6 6 3 3 3 3 3 3 3
: 1
a
1.5595 1.0839 1.4681 1.5493 1.0830 1.0847 1.0848 1.0839 1.1306
:
: 1 2 2 2 2 2 2 2 2 2
88.13 116.84 112.61 108.72 88.88 116.93 117.93 110.08 112.00 109.61 88.86 111.48 117.21 109.34 179.70
s
6
-17.2 -136.5 96.6 -138.2 102.5 -24.4 95.9 -23.4 -150.3 -48.9 180.0 17.3 137.4 -94.0 -95.4
a
9
i
a 9
24.7 153.3 137.5 -102.3 26.3
EQUATORIAL 1.5555 1.0846 1.4658 1.5498 1.0835 1.0842 1.0853 1.0836 1.1305 88.36 111.09 118.03 108.95 88.15 116.65 118.21 110.99 111.72 109.64 88.77 iii.58 117.18 109.29 179.21 -19.2 92.8 -140.3 -140.1 -28.0 98.8 93.4 -154.6 -27.7 52.1 180.0 19.3 138.7 -92.6 -93.5 26.0 154.6 139.4 -101.1
27.5
471
AXIAL CYANOCYCLOBUTANE
carbon and nitrogen following the procedure described in Ref. (I 0). Previous experience (12,23), based on a large number of studies, supports the expectation that the procedure applied will engender molecular structures for CYB which are sufficiently accurate for the current study. All geometries were optimized until the largest residual forces in the final cartesian coordinates were
DETAILS
CYB was prepared in accordance with the procedure described in Ref. (4). The spectrum was measured with a computer-controlled spectrometer in the frequency range 26-40 GHz with the cell thermostat at approximately -30°C. ASSIGNMENT OF THE ROTATIONAL
SPECTRUM
The rotational constants of both conformers were calculated from the ab initio structure. Then the calculated constants of the axial conformer were corrected with the differences found between the ab initio and experimental rotational constants of the previously assigned equatorial form.
TABLE II List of Measured Line Frequencies for the Axial Conformer of Cyanocyclobutane (MHz) J'(Ka.K,)
+
J”W,&)
4C1.3) - 3(0,3) 6(0.6) - 5(0,5) 6(1;5) - 5(1,4) 6(2,4) - 5(2,3) 6(3,31 - 5(3,2 1 6(5) - 5(5)a 7i1;71 - 6(1,6) 7t2.6) - 6t2.5) 7(3;5) - 6(3,4 7(4) - 6(4)a 1 7(6) - 6(6)a) 9(0.9) - 8C1.7) loi4;6j -1oi3;aj 11(4,7) -11(3,91 12t4.8) -12(3,10) 13(4,9) -13(3.11) 14(4,10)-14C3.12) 15(4,12)-15t3.12) 16(3.13)-16C2.15) 16(4;12)-16(3;14) 17(4,141-17(3,14) 18(4,15)-lE(3.15) 19(4,16)-19(3.16) 20(4.17)-2OC3.17) 21(4,18)-21t3.18) 22(4,19)-22t3.191 23(4,19)-23(3,21) a) Asymmetry
degenerate
J'(K,,K,)
26640.49 30967.11 31615.22 31264.84 31154.46 31139.31 35640.99 36277.84 36348.00 36337.52 36328.47 37319.42 34909.79 34897.61 34892.91 34902.31 34933.91 33622.22 30031.06 35108.16 32528.31 31789.38 30911.73 29893.67 28738.73 27455.31 38793.04 K-1
doublets.
+
J (Ka.Kcl II
5C1.4) - 4(0,4) 6t1.6) - 5(1,5) 612.5) - 5t2.4) 6C3.4) 5(3,3 1 6(41 - 5(4)a 7tO.7) - 6t1.5) 7(1,6) - 6(1,5) 7C2.5) - 6(2,4) 7(3,4) - 6t3.3 - 6(5)a i 7(5) 8CO.8) - 7(1,6) lO(4.7) -1OC3.7) ll(4.8) -11(3,8) 12C4.9) -1213.91 13(4;10)-13(3;10) 14(4,11)-14(3,11) 15(3,12)-15(2,14) 15(4,11)-15(3,13) 16(4,13)-16(3,13) 17(3,14)-17(2,16) 18(3.15)-18(2,17) 19(3,16)-19(2,18) 20(3.17)-20/2,19) 20(4,16)-20(3,18) 21(4,17)-21(3.19) 22(4,18)-22C3.20)
32293.18 30563.02 31105.81 31150.37 31142.87 28968.89 36864.14 36528.00 36357.69 36331.19 33239.18 34777.01 34666.14 34509.32 34293.05 34002.92 28848.84 34998.25 33135.69 31460.28 33158.45 35141.70 37419.71 36354.51 36979.05 37782.31
472
CAMINATI ET AL.
The first assigned transitions were the 73 + 63 and 63 + 53 Km1asymmetry doublets, which were measured with the radiofrequency microwave double resonance technique (14), pumping at about 6 MHz. They were found within a few tens of a MHz with respect to the calculated values. Other paR-type transitions were then measured with the conventional Stark modulation technique. Next it was possible to assign p&branch transitions, starting with 16(4, 12) + 16(3, 14) and 16(4, 13) + 16(3, 13), as they were radiofrequency pumped through the 16(4) K-i doublet at about 129 MHz. Several other pCtransitions were measured. According to the quadrupole coupling constants calculated by translating the values of the equatorial conformer (2) to the inertial axis system of the axial conformer, no quadrupole hypertme structure was observed, within our experimental resolution, for the measured transitions. All the measured transitions are listed in Table II. They have been treated with the Watson quartic reduced Hamiltonian (15), and the obtained rotational and centrifugal distortion constants are shown in Table III. Since the centrifugal distortion constants for the equatorial conformer were not determined in previous studies (I, 2), we extended the analysis of the microwave spectrum to this form. Table IV lists the frequencies obtained, and the corresponding spectroscopic constants are reported in Table III. The transitions 21,1+ lo,, and 22,0 +- 1r.0 reported in Ref. (I) deviated about 1 MHz when inserted in the fit, so they have been excluded. The latter, which falls in our frequency range, was overlapped by a much stronger transition. DIPOLE MOMENT
The displacements of the Stark lobes were measured for the 61,6 + 51,s and 71,6f 6 1,5transitions. The Stark coefficients (16) and the obtained dipole moment components TABLE III Spectroscopic Constants of Axial and Equatorial Conformers of CYB Axial
Equatorial
A/MHz
7591.86(6)
9761.88(3)
B/MHZ
2681.987(7)
2383.396(41
C/MHz
2505.943(8)
2105.118(3)
AJ/kHz
1.33(81 -3.31(4)
AJK/kHz
0.35(l) 0.96(13) 9.7(21)
AK/kHz
8.7(24)
bJlkHz
-0.126(Z)
0.016(21
dK/kHz
0.91(18)
1.6(2)
st. dev./MHz
0.12
"a' J max E a)
Number
60
23
19
258(50)
CO"f.fCm-'
of transitions
in the
fit.
0.15
53
0
TABLE IV List of Measured Line Frequencies for the Equatorial Conformer of Cyanocyclobutane (MHz) J’(K,,K,)
+ J”(K,,K,)
J*(K,.K,)
3lO.3) - 2(0,2) 3(1,21 - 2t1.11 3t2.1) - 2t2.0) 4(1,4) - 3t1.31 4(2,3) - 4(1,3) 5LO.51 - 4(0.4) 5i1;4j - 4ii;3j 611.5) - 511.41 6t2.4) - 5(2;3) 7f1.7) - 6t1.6) 7(2,6) - 7(1,6) 7t3.5) - 6t3.4) 7t3.4) - 6C3.3) 7(4;3) - 6i4;2jc) 7(6,1) - 6(6,O)C) 8C1.8) - 7t1.71 8C2.6) - 7i2.5) 8C3.5) - 7C3.4) 9(0,9) - 8(0,8) 9C2.8) - 9C1.8) lO(2.9) -10(1,9) 11(2.10)-ll(l,lO) ll(3.8) -11(2,10) 12(2,11)-12(1,11) 13(0,13)-1211.11) 14fO.14)-13f1.12) 16(2;15j-l5i3;13j 17(0.171-16(1,15) 17(3,14)-16(4,12) 19(0.19)-18t1.17) a) From ref. [23.
13434.64a) 13878.02a) 13496.35a) 17382.31a) 21175.88a) 222e9.72a) 23102.9ea) 27699.d’) 27159.99b) 30351.86b) 1e770.45a) 31475.29 31491.69 31457.18 31440.41 34656.09b) 36416.64b)
+
J”(K,,K,)
3(1,3) - 2(1,2) 3(2,21 - 2(2,1) 4(0,4) - 3CO.3) 4(1,3) - 3(1,21 4(2,2) - 3(2,1) 5(1,5) - 4(1,4) 5(2,3) - 4(2,2) 6(2,5) - 6(1,5) 7(0,7) - 6(0,6) 7(1,6) - 6(1,5) 7(2,5) - 6(2,4) 7(3,5) - 7(2,5) 7(3,4) - 7(2,6) 7(5,3) - 6(5,2)c) 8(0,8) - 7(0,7) 8(1,7) - 7(1,6) 8C3.6) - 7(3,51 8(3,5) - 8(2,7) 9(1,9) - 8C1.8) lO(O,lO)9(1,8) lO(3.7) -10(2,9) 11(3,9) -11(2,9) 12(0.12)-ll(l,lO) 12(3.10)-12C2.10) 13(3.11)-1312,li) X(3,13)-X(2,13) 16(3,14)-16(2,14) 17(2.16)-16t3.14) 18(0;18)-17(1;16) 19(3,16)-18C4.14)
$,$;;b) 16598:79a) 15376.36a) 14085.158) 39372.99 12746.65a) 33695.47 34985.73 29218.89 37884.38 27988.30 39464.83 b) From ref. Ill.
c, K-1
asymmetry
13043.26a) 13465.75a) 17e77.07a) 18494.64a) 18024.87a) 21714.36a) 22577.78b) 19694.26a)
3loo4.82b) 32283.09b) 31772.94b) 36899.04 37873.10 31446.10 353o3.35b) 36848.74b) 35983.87 38076.95 38949.65b) 28293.71 38796.56 34155.82 32176.42 33018.75 31702.53 28589.03 26828.45 32518.47 38682.97 39413.17 doublets
TABLE V Stark Coefficients and Dipole Moments of Axial CYB J'(Ka,Kc)
613
7l,6
+
J”(KasKc)
IMI
Av/E*
(Hz V-*
cm*,
exp.
talc.
1
-0.047
-0.048
2
0.154
0.159
3
0.507
0.504
4
0.999
0.988
+ 51.5
+ 61 . 5
1,2a)
'a
=
3.81(3)
-0.110
0
wb = 0.0 D (by symmetry) 1.34(3)
0
+* = 4.04(4)
0
PC
a) (MI = 1
=
and IMI = 2 component lines overlapped.
473
-0.107
474
CAMINATI ET AL.
for axial CYB are shown in Table V. The cell was calibrated at -30°C using OCS as standard. CONFORMATIONAL
EQUILIBRIUM
In Table VI the experimental rotational constants are compared with the values calculated from the ab initio structure of axial CYB. It is seen from this table that the new experimental constants are consistent with those calculated for the axial form. Line intensity measurements (17) were carried out on similar lines for the two conformers. The energy difference found between the two ground states is AE0,0 = E (axial)o,o - E (equatorial)O,O = 258 & 50 cm-‘. It is interesting to note that the differences between the rotational constants of axial and equatorial CYB, A, - A,, B, - B,, and C, - C,, are very similar for the experimental and the calculated constants obtained from the ab initio structures. These data are also included in Table VI. CONCLUSIONS
The material presented above allows the conclusion that we have succeeded in observing the microwave spectrum of a previously undetected conformer of monocyanocyclobutane and that the newly discovered transitions are consistent with the axial form of this compound. This finding is somewhat different from the previous microwave (I, 2) and infrared investigations (3) of this system. In line with our recent investigations of methylhydrazino carboxylate (5), 2-methoxy ethylamine (6), methylallyl alcohol (7), and glycine (8), this result emphasizes again the usefulness of highquality ab initio geometries in assigning weak rotational spectra in conformational mixtures. A particular advantage of the calculations is that they predict accurate dif-
TABLE VI Experimental and Calculated Rotational Constants and Shifts of Rotational Constants between the Two Observed Conformers of CYB (MHz)
A Axial
Equat.
ab initio
7591.86
7716.81
8
2681.987
2680.187
C
2505.943
2491.542
A
9761.88
B
2363.396
2373.945
C
2105.118
2102.39
AAa)
“AA=AAx
Exptl.
-2170.02
10282.69
-2565.88
AB
298.591
306.242
AC
400.825
389.152
. -A Eq. , etc.
AXIAL CYANGCYCLOBUTANE
475
ferences in local geometries (12, 13), i.e., conformationally dependent trends in bond distances and angles. For CYFJ, especially the axial form, we plan to obtain more extensive information on the ring-puckering potential surface, taking advantage of the results of the current investigation and exploring in greater detail the vibrational satellites associated with the ring motion. RECEIVED:
November
20, 1986 REFERENCES
1. 3. R. DURIG, L. A. CARREIRA,AND W. J. LAFFERTY,J. Mol. Spectrosc.46, 187-193 (1973). 2. M. Y. FONG AND M. D. HARMONY,J. Chem. Phys. S8,4260-4264 (1973). 3. C. S. BLACKWELL,L. A. CARREIRA,J. R. DURIG, J. M. KARRIKER,AND R. C. LORD, J. Chem. Phys. 56, 1706-1711 (1972). 4. M. DAKKOURI,H. EPHARDT,K. SIAM,L. SCHAFER,AND C. VAN ALSENOY,J. Mol. Struct., in press. 5. W. CAMINATI,A. C. FANTONI, L. SCHAFER,K. SIAM, AND C. VAN ALSENOY,J. Amer. Chem. Sot. 108,4364-4367 (1986). 6. W. CAMINATI,K. SIAM,J. D. EWBANK,AND L. SCI-LXFER, J. Mol. Struct., in press. 7. W. CAMINATI,K. SIAM, J. D. EWBANK, C. VAN ALSENOY,AND L. SCHAFER,J. Mol. Spectrosc.. in press. 8. L. SCHAFER,H. L. SELLERS,F. J. LOVAS,AND R. D. SLJENRAM, .I Amer. Chem. Sot. 102,6566-6568 (1980). 9. P. PIJLAY,Mol. Phys. 17, 197-204 (1969). 10. P. PULAY, Theor. Chim. Acta f&,299-312 (1979). II. P. PIJLAY,G. FOGARASI,F. PANG, AND J. E. Boccs, J. Amer. Chem. See. 101,2550-2560 (1979). 12. L. SCH@ER, J. Mol. Strut. 100,51-73 (1983). 13. L. SCHAEFER, J. D. EWBANK,V. J. KLIMKOWSKI,AND K. SIAM,J. Mol. Struct. 135, 141-158 (1986). 14. F. J. WODARCZVKAND E. B. WILSON,J. Mol. Spectrosc. 37,445-463 (1971). 15. J. K. G. WATSON,J. Chem. Phys. 46, 1935-1949 (1967). 16. S. GOLDEN AND E. B. WILSON, JR.,J. Chem. Phys. 16,669-685 (1948). 17. A. S. ESBITTAND E. B. WILSON,JR.,Rev. Sci. Instrum. 34,901-907 (1963).