J. Phys. Chem. 1981, 85,2655-2659
into internal excitation in HP Although the BO* emission observed under single-collision conditions may result from a metathesis involving highly translationally excited boron atoms, this is extremely unlikely in the multiple-collision studies where the boron atom is thermalized upon collision with argon to much lower temperature. Here, however, reactive encounters may be influenced by the presence of
2655
the argon atom as a third body and this may significantly effect the activation barrier.
Acknowledgment. It is a pleasure to acknowledge the comments of Professor W. H. Eberhardt on the first draft of this manuscript. Mr. A. W. Hanner aided this project by obtaining a portion of the spectra presented here.
Determination of Relative Signs and INDO-MO Calculations of lsC-15N Spin-Spin Coupling Constants of trans- and cis-Azobenzene and Benzo[ c ]cinnoline Yoshihiro Kuroda
laand
Yasuhlro Fujlwaralb
Facum of Pharmaceutlcal Sciences, Kyoto University, Sakyo-ku, Kyoto, 606,Japan, and Kyoto College of Pharmacy, Misasaglnakauchl-c~, Yamashina-ku, Kyoto, 607, Japan (Received: March 4, 1981; I n Flnal Form: May 14, 1981)
13C-lSNnuclear spin-couplingconstants of trans- and cis-azobenzene and benzo[c]cinnoline in neutral and strong acid solutionsare reported. Relative signs of "J(C-N) and the corresponding "+'J(C-N) (n = 1-4) were determined by comparing the 13C signal splittings of the lsN2compound with those of the corresponding 16N1compound. The geometrical difference of the nitrogen lone-pair disposition between trans- and cis-azobenzene or benzo[c]cinnolineis clearly reflected in one- and two-bond coupling constants. INDO-MO calculations of "4C-N) were performed to determine whether they can correctly reproduce the absolute magnitudes and the relative signs of V(C-N). It was found that, although the INDO-MO calculations gave reasonable values for one-bond couplings,they tended to give too positive values for two-bond couplings and too negative values for three-bond couplings. 16N isotope effects on 13C chemical shifts are also reported.
Introduction Carbon-nitrogen (l3C-lSN)spin-spin coupling constants, "J(C-N), have been extensively investigated not only because of their chemical importance as NMR parameters but also because of their interesting variations in sign and in absolute magnitude with respect to hybridization states and lone-pair orientations of interacting nitrogen atoms.2 In the previous study on the protonation site of 4-aminoazobenzene by 16N and 13C NMR spectro~copy,~ we reported some 13C-16Nspin-couplingconstants together with the theoretically calculated values using an INDO-MO level of approximation. The observed changes in V(C-N) on protonation made it possible to determine protonation sites of 4-aminoazobenzene in strong acid media. The calculated values agreed well with the observed trends, especially for one-bond couplings. INDO-MO calculations also predicted reasonable dependence on the molecular geometry of one-bond couplings. However, some questions remained as to the calculated protonation and molecular geometry dependence of two-bond couplings, since no signs of V(C-N) had been determined experimentally. In general, the experimental determination of the absolute sign of "J(C-N) is a difficult task, and only a few examples have been r e p ~ r t e d . Therefore, ~ we usually refer to the(1) (a) Kyoto University; (b) Kyoto College of Pharmacy. (2) (a) Levy, G. C.; Lichter, R. L. "Nitrogen-I5 Nuclear Magnetic Resonance Spectroscopy";Wiley-Interscience: New York, 1979;Chapter 4. (b) Wasylishen, R. E. Annu. Rep. NMR Spectrosc. 1977, 7, 245-91. (3) Kuroda, Y.; Lee, H.; Kuwae, A. J . Phys. Chem. 1980,84,3417-23. (4) (a) Bundgaard, T.; Jakobsen,H. J. TetrahedronLett. 1974,1621-4. (b) Buchanan, G. W.; Dawson, B. A. Can. J. Chem. 1977,55,1437-9. (c) Hansen, M.; Jakobsen, H. J. Acta Chem. Scand. 1972,26, 2151-3. 0022-365418112085-2655$0 1.2510
oretically calculated values as a sole clue to the signs.6 One possible way to determine experimentally a sign of V(C-N) is to use the Jakobsen technique.6 This technique can determine the relative sign of "J(C-N) and the corresponding n+lJ(N-H) by applying off-resonance or a selective proton decoupling technique to the observed 13C multiplet due to V(C-H). In this work, we have employed another approach to obtain information on the signs of V(C-N) of aromatic azo dyes, including quarternary carbons. In this method we analyze second-order spin systems of 13Cresonances which are spin-coupled to 16N nuclei. In the second-order spin systems, if we can pick out two signals the separation of which is equal to the sum of two coupling constants and, more importantly, can violate the second-order spin-coupling state to give a firstorder spin-coupled spectrum by some means (for example, by an isotopic substitution), a relative sign between the coupling constants can be obtained by simple mathematical manipulation of the sum and each absolute value.' This method can be applied to relatively simple com(5) For example, calculated values for V(C-N) including Fermi contact, orbital-dipole,and spin-dipolar terms of various kinds of molecules are shown in the following literature: (a) Schulman, J. M.; Venanzi, T. J. Am. Chem. SOC. 1976, 98, 4701-5. (b) Khin, T.; Webb, G. A. Org. Magn. Reson. 1977,10, 175-8. (c) Khin, T.; Webb, G . A. Ibid. 1978,11, 487-92. (6) (a) Jakobsen, H. J.; Bundgaard, T.; Hansen, R. S. Mol. Phys. 1972, 23, 197-201. (b) Sorensen, S.; Hansen, R. S.; Jakobsen, H. J. J. Am. Chem. Soc. 1973, 95, 5080-1. (7) If we can observe and analyze the corresponding 16NNMR spectrum (XX' part) by using 'T-enriched material at both benzene rings, the relative sign can be determinedwithout reduction of the second-order coupling state to the first order. See, for example: Pople, J. A.; Schneider, W. G.; Bernstein, H. J. "High-Resolution Nuclear Magnetic Resonance"; McGraw-Hill: New York, 1959; pp 130-8.
0 1981 American Chemical Society
2656
The Journal of Physical Chemistry, Vol. 85, No. 78, 7987
Kuroda and Fujlwara
trans-Azobenzene
N=d /
(H)
Q3
,
I
140
150
5 4 c is-Azobenzene
130
1
l*Oppm
Flgure 2. ’‘C NMR spectra of frans-a~obenzene-~~N, In CDCI,. Assignments and the separations (in Hz) between two adjacent lines are given on top of each 13C resonance.
5
T1T
4
Benzo[c]cinnoline
Flgure 1. Numbering of ring carbons in frans- and cis-azobenzene and benzo[c]cinnohe. The numbering with prime is also used to distlnguish the two benzene rings when one of the nitrogens is protonated or “N enriched.
pounds which have magnetically high symmetry to cause second-order couplings. Among various aromatic azo dyes, azobenzene-15Nz (Figure 1)satisfies this requirement, and the spin system of the 13Cresonances can be analyzed as an AXX’ system (A for natural abundance 13C,and XX’ for 15N-enriched azo nitrogens). The signal pattern of the A part is expected to be a triplet, the separation between which corresponds to half the sum of two coupling constants, i.e., l/z(nJ(C-N) n+1J(C-N)).8 The absolute value for each constituent may be obtained from the 13C spectrum of the 15N1compound. In the present paper, 13C-15N spin-coupling constants of trans- and cis-azobenzene and benzo[c]cinnoline which has a “fixed” geometry as a cis isomer of azo dyes (see Figure 1)are reported together with relative signs of various sets of WC-N) and *+lJ(C-N). INDO-MO calculations are carried out to see whether the relative signs are reproduced correctly.
+
Experimental Section Materials. Azobenzene (Nakarai Chemicals, Ltd., Kyoto) and azobenzene-l5Nz(95% enriched, CEA-CENSaclay) were purified by column chromatography on alumina with petroleum ether. Azobenzene-15N1was synthesized from anilineJ5N1 (95% enriched, CEA) and nitrosobenzene (Aldrich) as described by Holt and Hughes? The cis isomer of azobenzene was prepared according to methods described in the literature.1° Benzo[c]cinnoline-l5Nzand its I5N1compound were prepared from a~obenzene-~~N, and azobenzene-15N1,respectively, as described by Badger et al.ll Measurements. The 13C NMR spectra were recorded on a Varian CFT-20 FT NMR spectrometer operating at 20.0 MHz and under conditions of complete proton de(8)Abraham, R. J.; Bernstein, H. J. Can. J.Chem. 1961,39,216-30. Our AXX’ system corresponds to the ABX system in this reference (XX’
to AB and A to X). (9)Holt, P.F.;Hughes, B. P. J. Chem. Soc. 1953,1666-9. (IO) See, for example: Leuenberger, C.; Karpf, M.; Hoesch, L.; Dreiding, A. S. Helu. Chirn. Acta 1977,60,831-43. (11)Badger, G.M.;Drewer, R. J.; Lewis, G. E. Aust. J. Chem. 1963, 16,1042-50.
Y
U 1
150
,
140
!I!
L ,
130
120 ppm
Flgure 3. 13C NMR spectra of trans-azobenzene-16Nl in CDCl3. Assignments and the separations (in Hz) between two adjacent lines are given on top of each 13C resonance.
coupling. The spectra were taken at a pulse width of 7-9 flip angle) with 8 K of memory for 40001000-Hz spectral width. To ascertain the pairing of 13C lines for spin-coupled doublets, 25.05- or 50.3-MHz 13C NMR spectra were also measured on a JEOL FX-100 or a Varian XL-200 FT NMR spectrometer. As neutral solutions, 1.4-0.8 mol dm-3 solutions of the azo dyes dissolved in CDC13were employed for the measurements. As strong acid solutions, 0.7 mol dm-3 solutions of the azo dyes dissolved in 22 N DZSO4-EtOH(7:1, v/v) were employed for the measurements. Chemical shifts were determined from internal tetramethylsilane for the neutral solutions and from external dioxane (as 67.4 ppm) dissolved in DzO, which was contained in a coaxial capillary tube, for the strong acid solutions. INDO-MO Calculations of Spin-Spin Coupling Constants. The method of calculation was the same as in the previous work,3 i.e., self-consistent perturbation calculations at the INDO-MO level of approximation.lZ Onecenter integrals were taken from those calculated by Towl and Schaumburg by the use of Slater e~p0nents.l~The molecular geometry of trans-azobenzene was taken from gaseous-state values,14 and that of the cis isomer was constructed from structural parameters of the trans isomer by changing the dihedral angle of C-N=N-C and the angle of C-N=N to those of cis-azobenzene reported in p s (30-45’
(12)(a) Blizzard, A. C.; Santry, D. P. J. Chem. Phys. 1971,55,950-63. (b) Blizzard, A. C.; Santry, D. P. Ibid. 1973,58,4714. (13)Towl, A. D. C.; Schaumburg, K. Mol. Phys. 1971,22,49-66. (14)Traetteherg, M.;Hilmo, I.; Hagen, K. J. Mol. Struct. 1977,39, 231-9.
The Journal of Physical Chemistry, Vol. 85, No. 18, 1981 2657
IsC-''N Spin-Spin Coupling Constants of Azobenzene
TABLE I: Observed ,C-l 5N Splittings and Benzo[c]cinnoline in CDC1, carbons
152.8 122.9 129.1 130.9
1
153.6 120.5 128.7 127.3
1 2 3 4 5 6
145.2 131.2 129.2 131.4 121.3 120.8
In ppm from Me,Si, t O . l ppm. Unreliable.
a
Chemical Shifts of trans-Azobenzene, cis-Azobenzene, and signal splittingsb
chemical shiftsa
1 2, 6 3, 5 4
2, 6 3, 5 4
I3C
"N, trans-Azobenzene 1.7, 1 . 7 2.0, 1.9, 1.5 4.0, 4.0 3.9 0.9, 0.8 1.0, 1.0 (0.4)' 0.8 cis-Azobenzene 6.9, 6 . 9 2.8, 3.9, 3.3 2.5, 2.5 2.5
(0.8p (0.6)'
' J = + 1.9, '5' - 5.4 'J=-3.9, ,J=-3.9 ' J = t2.0, 4J= ~ 0 . 3 ~ 4J= k0.8, sJ= ~ 0 . 8 ~ ' J = + 3.9, ' J = + 10.0 'J=-2.5, ,J=-2.5
(1.1)' (0.8)'
Benzo[c] cinnoline 5.5, 5.4 1 . 6 , 3.4, 2.3 6.3, 6.4 3.4, 3.1, 3.2 1 . 7 , 1.8 1.6, 1 . 5 (0.6)' 0.8 ( 0.8)c (0.4)' 1.9, 1.8 2.0, 0.8, 1.7 In Hz (at 20.0 MHz), i 0 . 2 Hz.
'J= +3.4, ' 5'
+7.3
' J = -9.7, 3 J = -3.1 'J= 73.5, 4 J = t 0 . 4 4 J = i0.8,5 J = ~ 0 . 8 ~
' J = +4.5, 3J=-0.8
Unresolved, Half-height line width is shown here.
Results and Discussion Spin-Spin Coupling Constants in Neutral [CDC13] Solutions. Figures 2 and 3 show 13CNMR spectra of trans isomers of azobenzene-15N2and azobenzene-15N1,respectively. As expected, all of the resolved 13Cresonances of the 15N2compound consist of triplets resulting from the spin-spin interactions with the two 15Nspins. On the other hand, the 13C NMR spectrum of the 15N1compound consists of three kinds of splitting patterns, that is, a quartet (Cl), a triplet (C3,5), and doublets (C2,6 and C4). In this case, since only one of the nitrogens is 15Nenriched, coupling to the 15Nspin should be simply additive rather than multiplicative. Thus, two pairs of AX-type doublets are generally expected for each 13C resonance. The doublet patten in Figure 3, therefore, means that absolute magnitudes of "J(C-N) and the corresponding 'W((2-N) are approximately equal, while the triplet pattern means that either of the coupling constants is very small and the separation between the outer lines is equal to the remaining coupling constant. This latter interpretation is reasonable since the 13C nuclei showing a triplet pattern (C3,5) are located fairly far from the azo nitrogens (n = 3,4). In Table I observed signal splittings of 15N2and 15N1 compounds are summarized together with the chemical shifts and assignments1' for each 13C resonance. The values are mere intervals between adjacent lines and do not always directly lead to the absolute values of spincoupling constants. For unresolved signals, half-height line widths are shown in parentheses. The observed splittings are so arranged that a more left-side value corresponds to a lower field interval between adjacent lines (see, for example, C1 resonance in Figure 3). The signal splittings of cis-azobenzene and benzo[c] cinnoline are likewise summarized in Table I, together with the 13C chemical shifts and assignments. The assignments were determined by taking into account the expected chemical shifts, the results of off-resonance proton decoupling experiments,
magnitudes of signal splittings of 15N2and 15N1compounds, and protonation behavior of these parameters.18 All of the observed splittings give us pairs of coupling constanta, "J(C-N) and "+lJ(C-N), for each 13Cnucleus together with the relative signs between them. For example, absolute values of 'J(C1-N) and V(C1-N) of trans-azobenzene are determined to be 1.9 and 5.4 Hz, respectively. The former simply corresponds to the separation between the inner lines of the quartet and the latter to that of the outer lines (Le., the sum of each separation of the 15N1compound). It is of interest here that the two outer separations of the quartet are not equal (2.0 vs. 1.5 Hz). This discrepancy seems to arise from a slight chemical shift difference of 13Cresonances between benzene rings bonded to 15Nand to 14Nand can also be seen for other quartets (C1 of the cis isomer and C1 and C2 of benzo[c]cinnoline). We assigned ' J and 2J for the following two reasons: (1) The magnitude of 2J(C-N) may compare with that of the corresponding coupling of 4-aminoazobenzene (5.0-5.5 H z ) . ~(2) The magnitude of lJ(C1-N) may be small because of a one-bond lone-pair effect5" as in the case of 4-aminoazobenzene. From the comparison of absolute magnitudes of lJ and 2J with the splittings of the corresponding 15N2compound, it follows that they are of opposite sign. Absolute magnitudes and relative signs of other couplings were determined in a similar manner and are summarized in the last column of Table I. The assignments of "J and "+lJ (that is, which of the doublets should be selected as " J , or "+lJ) were determined by taking into account the magnitudes of splittings, lone-pair orientations, and the s character of lone-pair orbitals for the one-bond lone-pair effect. In cases in which we could estimate absolute signs by analogy with other compounds (especially with (E)-and (2)-acetophenone oxime,4band pyridine and its derivatives4") and also from the rule so far established that a lone pair cis to the coupled carbon makes a negative contribution to 2J(C-N),2-4 the signs are indicated. If the sign estimation is impossible, the following notations are employed: for the same sign, &"J and fn+lJ,and for the opposite sign, *"J and V+lJor r"J and
(15) Mostad, A.; Ramming, C. Acta Chem. Scand. 1971, 10, 3561-8. (16) Van der Meer, H. Acta Crystallogr., Sect. B 1972, 28, 367-70. (17) Johnson, L. F.; Jankowski, W. C. "Carbon-13 NMR Spectra"; Wiley-Interscience: New York, 1972; spectrum no. 431.
(18) Regarding the benzo[c]cinnoline, thus determined assignments differ from those reported by Kooti and Nixon (Kooti, M.; Nixon, J. F. J. Organomet. Chem. 1976,105,217-30). They showed no reasoning or the method of assignments.
the 1iterat~re.l~The molecular geometry of benzo[c]cinnoline was taken from the results of X-ray analysis.16
2658
The Journal of Physical Chemistry, Vol. 85, No. 18, 1981
TABLE 11: Calculated and Experimental Coupling Constants (Hz) nJ(C-N)
J(expt1)
FCa
OBa
SDa J(total)
'J(C1-N) 1J(C2,6-N)b 3J(C2,6-N)b
trans-Azobenzene +1.9 1.1 0.6 -5.4 3.0 - 0 . 4 -3.9 5.3 - 0 . 5 -3.9 -7.2 -0.5
-1.5 1.0 0.7 -0.6
0.2 3.6 5.5 -8.3
J( C1-N) 'J(C1-N) 'J(C2,6-N)b 3J(C2,6-N)b
cis-Azobenzene +3.9 4.4 0.8 t10.0 0.1 6.9 5.2 - 0 . 2 -2.5 -2.5 -7.3 -0.2
- 0.5 0.1 -0.2 0.4
4.7 7.1 4.8 -7.1
'J( C 1-N)
J( C 1-N ) *J(Cl-N) ' J ( C 2-N) 'J(C2-N) 'J( C6-N) 3J(C6-N)
Benzo[c] cinnoline +3.4 4.0 1.8 +7.3 5.1 -0.1 -9.7 -0.1 -0.4 -3.1 -3.5 -0.5 +4.5 4.4 - 0 . 6 -0.8 -0.5 -6.3
-1.0 0.9 0.8 -1.0 0.8 -1.2
4.8 5.9 0.3 -5.0 4.6 -8.0
' FC = Fermi contact term; OB = orbital-dipole term;
SD = spin-dipole term. The parameters employed were S,'(O) = 5.246, S ' ( 0 ) = 3.012, ( r - j ) ~= 2.472, and (r-3)c = 1.430.13 Calculated values are averages of those of C2 and C6. For example, each value of C2 and C6 for 'J(C2,6-N) of trans-azobenzene is as follows: C2, (FC, OB, S D ) = ( 3 . 7 , - 0 . 3 , 0 . 8 ) ; C 6 , (FC, OB, S D ) = ( 6 . 9 , - 0 . 7 , 0.7).
f"+'J.We have estimated the absolute sign of %J(C2,6-N) of cis-azobenzene to be negative, since its absolute magnitude closely matched a mean value of 2J(C2-N) and V(C6-N) of benzo[c]cinnoline (Le., 1/2(-9.7 + 4.5) = -2.6 Hz). In Table I1 13C-15N spin-coupling constants calculated according to the Blizzard and Santry method12are compared to the observed ones. It can be seen from this table that the calculations do not always correctly predict the relative signs within each set of coupling constants. Moreover, the calculations tend to give too positive values for two-bond couplings and too negative values for three-bond couplings. The former trend was also pointed out by Wasylishen in the case of quinoline,l9 although in this case the estimated sign itself is correctly predicted, in contrast to our results. In the benzo[c]cinnoline, for example, the absolute sign of V(C2-N) can be estimated to be negative (-9.7 Hz) owing to proximity to the nitrogen lone pair as in the case of quinoline (2J(C8-N) = -9.3 H z ) , ~ ~ whereas the calculation predicts a positive value (+0.3Hz). In spite of these disappointments, the calculations predict reasonably well the effects of lone-pair orientation and the degree of s character of the lone-pair orbital on two-bond and one-bond couplings, respectively. For example, as mentioned above the 2J(C2-N) of benzo[c]cinnoline is expected to be more negative than ?-J(C6-N). This is, in fact, predicted as shown in Table 11. The larger value of lJ(C1-N) in cis-azobenzene (or benzo[c]cinnoline) compared to that in the trans isomer may be due to the increased s character of the nitrogen lone-pair orbital which is known to make a positive contribution to the Fermi contact (FC) term.5a This trend is clearly reproduced by the calculations. Furthermore, the calculations correctly predict the relative signs of lJ(C1-N) and 2J(C1-N) of cis-azobenzene and benzo[c]cinnoline. It might also be noted that ?-J(Cl-N)of cis-azobenzene (and also of benzo[c]cinnoline) is relatively large compared to that of the corresponding trans isomer for its trans orientation of lone (19)See p 277 of ref 2b.
Kuroda and Fujiwara
TABLE 111: Observed 13C-1sNSplittings and 13C Chemical Shifts of trans-Azobenzene and Benzo[ c ] cinnoline in Strong Acid Media car- chemical bons shifts' 1
143.8
2, 6
126.4
3, 5
4
131.9 139.7
1
139.8
2
126.9
3 4 5 6
133.9 138.1 122.7 126.7
signal splittingsb 15N,
15N1
trans-Azobenzene 4.0. 4.1 1.6. 0.8. 4.1 ' 3.1, 3.4 3.3 (4.1)' (4.2)'
J( c-N ) b 'J=-7.1. ' J - 0.8 ' J = - 3.3, 3J= -3.3
(4.0)' (4.4)'
Benzo[c] cinnoline (1.5)' 1.7, 2.0, 2.6 4.5, 4.5d (0.9, 1.8), 4.5e (5.0)' 1.6, 1 . 5 (2.5)' (3.1)' (2.5)' (3.0)' 1.8, 1 . 8 1.6, 2.3
' J = -3.7, 'J = + 4.6 'J=-7.2, 3J= -1.8 ' J = 7.3.1
' J = +0.3,f 3 J = -3.9
a In ppm from external dioxane (as 67.4 ppm), k O . 1 ppm. Measured in 2 2 N H,SO,-EtOH (7:1, v/v). In Hz (at 20.0 MHz), i 0 . 2 Hz. Measured in 22 N D,SO,EtOH ( 7 : 1 , v/v). ' Unresolved. Half-height line width is shown here. Overlapped with C6 triplet. e Measured in 1 5 N D,SO,-EtOH ( l O : l , v/v), so as to avoid peak overlap with C6 resonance. (0.9, 1.8): Since two lower field signals of this C2 quartet were not resolved, these values were estimated from the half-height line width and the peak separations between the unresolved signal and the remaining higher field signals. Unreliable.
pairs against C1; here again the calculations agree well with the trend. All of these findings lead us to conclude that INDO-MO calculations can predict trends correctly; however, care is required in the use of the calculated values for determining absolute signs and absolute magnitudes except one-bond couplings. Spin-Spin Coupling Constants in Strong Acid Solutions. It is interesting to investigate the protonation effect on 13C-15N spin-coupling constants since the protonation greatly alters the nitrogen lone-pair orbital. Observed signal splittings and chemical shifts of trans-azobenzene and benzo[c]cinnoline are summarized in Table 111. The assignments of 13C resonances were determined on the basis of magnitudes of splittings and protonation shifts of 4-aminoazobenzene and also by referring to the calculated changes in total charge densities on protonation.20 We could not observe the 13CNMR spectra of cis-azobenzene because of its rapid conversion to the trans isomer within a few minutes.21 On account of viscosity and/or proton exchange broadening, only a few resonances could be resolved. From the observed splittings, each set of coupling constants was derived, and these are shown in the last column of Table 111,where absolute signs and the assignments of "Jand "+lJ were determined in accordance with the following reasons: (1)lJ should become larger than in neutral solution and also negative in sign because of release from the suppression of the one-bond lone-pair (2) 2J(C6-N) of benzo[c]cinnolinemay be smaller than 3J(C6-N) and may be positive in sign by analogy with the cases of pyridinium and protonated quinoline.2b (3) 2J(C2,6-N) of trans-azobenzene may be comparable to a mean value of 2J(C2-N) and %J(C6-N) of benzo[c](20) This result will be published elsewhere. (21) In neutral (CDC13)solution, the cis isomer maintained its conformation for 20-30 h.
13C-1bN Spin-Spin Coupling Constants of Azobenzene TABLE IV: Observed Chemical Shift Differences (A6 ) between C1 Bonded to I5N and C1’Bonded t o 14Nb
compd
A& in neutral solutionsa
A6 in strong acid solutionsa
trans-azobenzene cis-azobenzene benzo [c ] cinnoline
-0.2, +0.2, + 0.3,
-1.5,
The Journal of Physical Chemistry, Vol. 85, No. 18, 1981 2659
TABLE V: Calculated and Experimental Coupling Constants (Hz) “J(C-N)
FCa
OBa
SDa
J(tota1)
trans-Azobenzene ‘J(C1-NH)
-14.2
0.7
-0.3
-13.8
-1.0 1.5
1.6 0.2
-0.4 0.2
0.2 1.9
6.2
0.1
0.3
6.6
-0.2
-12.2
-0.3 0.4
1.3 3.4
0.4
5.5
-7.1 ‘J(Cl’-N) *J(C 1-N )
(+)2.1,
-0.8
The positive value means that the C1 bonded to I5N In Hz at 20.0 MHz. resonates at lower field (+0.2Hz). a
cinnoline (Le., ‘/2(-7.2 + 0.3) = -3.5 Hz). This should also be noted in determining lJ and 2Jof benzo[c]cinnoline. The C1 resonance of the 15N2compound shows no splittings, while that of the corresponding 16N1compound splits into a quartet (Table 111). This result means that the I J and 2Jof protonated benzo[c]cinnoline are of opposite sign. However, we cannot choose the separation between the inner lines of the quartet and that between the outer lines of the quartet as I J and 2J,respectively. If we choose in this way, since I J + 2J= +4.3, we cannot explain why the I3C spectrum of the I5N2compound shows a singlet signal. This discrepancy suggests that we should assign other pairs of lines to lJ and 2J. In order to determine the pairing of 13C lines for the spincoupled doublets, we measured the 25-MHz 13Cspectrum of the 15N1compound and found that we should pick out the quartet lines alternately. This assignment results in (lJ,2J) = (-3.7, +4.6) or (-4.6, +3.7), and the sum of lJ and 2Jbecomes hO.9 Hz, the magnitude of which lies well within a half-height line width of the singlet peak. This alternate choice of quartet lines for I J and 2Jmeans that the chemical shift difference between the two quarternary carbons (Cl and Cl’) increased upon protonation. In order to see this situation quantitatively and to compare the magnitude with the other compounds, we calculated all of the chemical shift differences (A6) from the observed splittings of C1 resonances. The calculations were made by considering that the C1 doublet which was assigned to give one-bond coupling is a carbon bonded to 15N. Table IV lists the values in Hz (at 20.0 MHz) from which the following can be seen: (1)In neutral solutions, the C1 bonded to 16Nresonates a t a higher field than the C1’ bonded to 14N in the case of trans-azobenzene, whereas the situations are reversed in the cases of the cis isomer and benzo[c]cinnoline. (2) On protonation, A6 of trans-azobenzene increases in magnitude (ca. 6 times) and also the C1 bonded to 15N resonates at higher field than Cl’. (3) On protonation, A6 of benzo[c]cinnoline also increases in magnitude and t o the same extent (that is, ca.
J(expt1)
*J(C l‘-NH) ‘J(C1 -NH) ‘J( C1’-N) ‘J(C 1-N )
’J(C 1’ -N H ) a
Benzo[c] cinnoline -13.6 1.6 -3.7 -1.0 2.6 1.9 1.1 +4.6 5.1 0.0
See corresponding footnote of Table 11.
6 times) as that of the trans-azobenzene. Here, the sign may be estimated to be positive by analogy with that in neutral solution. This estimation of sign results in (lJ,2J) = (-3.7, +4.6). The reasons for the sign alteration in A6 between trans and cis azo dyes and the increase of A8 upon protonation are not obvious and will require further investigation. Finally, in Table V, l3CJ6N spin-coupling constants concerning C1 determined thus are compared with the calculated values. The INDO-MO calculations were made on the assumption that only one proton is attached to a nitrogen to a distance of 0.998 A. It was also assumed that the protonation does not affect the overall molecular geometry. Since there are two nitrogens with equal possibilities for protonation, each experimental value can be regarded as a mean value reflecting alternate protonation. This situation is successfully reproduced in the calculations for one-bond couplings of trans-azobenzene and benzo[clcinnoline and two-bond coupling of benzo[c]cinnoline. In the case of the two-bond coupling of trans-azobenzene, again too positive values are predicted by the calculations. Incidentally, observed 2J(C1-N) and 2J(C1’-N) of 4aminoazobenzene are (-)2.5 and 0.0 Hz, respe~tively.~ The mean value compares favorably with -0.8 Hz of transazobenzene, supporting our present assignment for the and 2J.
Acknowledgment. We thank Dr. M. Sugiura of Kobe Women’s College of Pharmacy and Miss K. Kinoshita of Kyoto University (Institute for Chemical Research) for the measurements of 50- and 25-MHz 13CNMR spectra. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan.
