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Quadrupole coupling in 1-d-pyrazole and 4-d-pyrazole principal quadrupole coupling constants for pyrazole
Journal of Molecular Structure
Elsevier Publishing
QUADRUPOLE PRINCIPAL
.Company, Amsterdam.
COUPLING QUADRUPOLE
G. L. BLACKMAN.
R. D. BROWN,
465
Printed in the Netherlands
IN l-D-PYRAZOLE COUPLING
F. R. BURDEN
AND 4-D-PYRAZOLE
CONSTANTS
AND
FOR PYRAZOLE
A. MISHRA
Department of Chemistry, Monash Uniuersity, Clayton, Victoria (Australia)
Pth,1970)
ABSTRACT
The hyperfine structure of a number of lines in the microwave spectra of 1-D-pyrazole and 4-D-pyrazole has been measured and analysed, First order quadrupole coupling theory for the two nitrogen atoms has been used to generate computer simulated multiplets for optimal fitting to the corresponding spectrometer tracings. Fittings to all multiplets have been achieved to within the observational limits. By combining the derived coupling constants with those obtained previously for pyrazole itself and assuming that the field gradient tensors are unaltered by isotopic substitution, two independent and closely concordant sets of values have been obtained for the full field gradient tensors at each of the two nitrogens. The results have been compared with field gradient tensors calculated from molecular orbital wave functions. Reasons for the unsatisfactory performance of the CNDO/~ wavefunction are advanced and some implications of the present experimental values for the electronic structure of pyrazole are mentioned.
INTRODUCTION
There have been many attempts to use experimental quadrupole coupling constants for nitrogen nuclei in heterocyclic compounds to obtain greater insight into the electronic structures of such molecules’- ‘. However, for a number of interesting cases the relevant coupling constants, referred to principal field-gradient axes, have hitherto been available only from NQR observations on solids, where the contribution of lattice effects is appreciable and uncertain. The present work was aimed at obtaining the relevant off-diagonal coupling constants in the inertialaxis system of pyrazole so that our previously reported coupling constants foi the gas phase4 could be transformed to the principal-quadrupole axis system. J. Mol. Structure, 9 (1971) 465-473
466
G. L. BLACKMAN,
H (31
Fig. 1. Direction those of pyrazole
R. D. BROWN,
F; R. BURDEN,
A. MISHRA
b
of inertial axes of I-D-pyrazole itself.
(a”, 6”) and 4-D-pyrazole
(a’, 6’) relative to
As the observed hyperfine structure of microwave lines of nitrogen heterocycles can be completely accounted for by a first-order quadrupole coupling theory (i.e. second order effects are too small to be measured so far) the off-diagonal elements must be found by an indirect method. The method previously used’ involves the determination of coupling constants, referred to inertial axes, for two isotopic species such that the direction of inertial axes relative to the nuclei differ appreciably in the two species. We report measurements on I-D- and 4-D-pyrazole, for which the angles of rotation of the inertial axes relative to those in pyrazole itself have been obtained6. They are indicated in Fig. 1.
EXPERIMENTAL
The microwave spectrometer used for this work has been described previousIy4. The needle-shaped, colourless crystals of I-D-pyrazole were collected by J. Mol.
Strucfwe,
9 (1971)
465-473
QuADRUPOLE
COUPLING
IN
I-D-PYRAZOLE
AND
4-D-PYRAZOLE
467
recrystallization from a solution of pyrazole in heavy water. 4-D-pyrazole .was prepared by the method described by Chung and Vaughan’. The rotational constants for both I-D- and 4-D-pyrazoles together with the angles of rotation of. the principal inertial axes on deuteration have been obtained by Nygaard6. All spectra were taken at low pressures and dry-ice temperature in order to achieve maximum resolution. Table 1 lists a11 measured lines for the two deuterospecies of pyrazole. TABLE
1
OBSERVED
AND
CALCULATED
HYPERFINE
TRANSITION
FREQUENCIES
Rotational
Hype&e
Frequency
transition
transition (FI_ --+ F’I,12)
Observed
22-+ 12
13432. I 1
(A)
(MHZ) Calculated
Dtfference
13432.12
0.01 b b b b b b
I-D-Pyrazole 0 00 -+ 101 (v,, = 13432.60 MHz)
11+2, 22 - 32 00--f lo I 1x+01 2L + 22 22+ 11 1
000 -+ 111
(vg = 14028.14 MHz)
22 -
13432.58
13432.82
14027.06
12
11-21 22 +
14028.04
32 1
OO-fll 11+01 22
+
14028.63
22 I
22’10
13432.48 13432.57 13432.59 13432.74 13432.83 13432.92
14027.06 14027.97 14028.05 14028.08 14028.59 14028.67 14028.74
0.00 b b b b
14646.02 14646.13 14646.28 14647.49 14647.71 14648.17
b b b 0.01 0.01 0.01
12290.24 12290.29 12290.41 12291.12 12291.16 12291.32 12291.48 12291.62
b b 0.01 b b
212 + 221 ho =
14647.18 MHz)
ll-tll
14646.13
32 -+ 32
312 -+ 321 (v. = 12290.98 MHz)
22
-+
22
42
+
42
31+
31
2x4
2,
21
21
+
1
42 -+ 4t I 32 32 31 -+ 30 1 52 52 I 41 + 41 30 --f 31 22 -+ 22
14647.38 14647.70 14648.16
12290.29 12290.40 12291.17 12291.30 12291.48 12291.58
J. Mol.
Structure,
b”
0.02 0.00 0.04
9 (1971) 465473
468
G. L. BLACKMAN,
TABLE
R. D. BROWN,
F. R. BURDEN,
A: MISHRA
1 (continued)
Rotational transition
Hyper-ne transition (FIZZ + F’I,,,)
Frequency (MHz) nbserrted
Caicrdated
Diference
13149.03 13149.19 13149.38 13150.30 13150.38 13150.57 13152.29
0.00 0.00
0.00 b’ b 0.01 0.01
16608.20 16608.59 16608,69 16608.82 16609.18 16609.28 16610.32 16610.67 16610.81
0.02 b b b -0.01 b -0.01 0.00 b
14689.62 14689.64 14690.01 14690.09 14690.14 14690.49 14692.10 14692.19 14692.64
b b b b b 0.00 b b 0.00
14096.69 14097.88 14098.26 14098.66 14099.40 14099.52 14099.92
0.00 -0.01 -0.01 0.00 b -0.01 0.02
(B) 4-D-PyrazoIe 0
-
lo1
13150.21 MHz)
(i," =
13149.03’ 13149.19’ 13149.38=
22-+10 22
+-22
ll-+Ol
13150.36
oo+-11 22
-32
1
22'12
13150.56* 13152.28”
h--+22
16608.185
31 + 31 41 41 12 + + 12 I
16608.65
11+&
322 (v,,
--;’ 331 = 16609.43 MHz)
52
16609.19”
+52
30 + 32 + 41 21 -
30 32 42 21
16610.33” 16610.67”
Ll --f 550
(v, = 14690.80 MHz)
32 -+ 32 51 -+ 5‘ I 50 -+- 50 41-+4t 72 72 I
14689.63 14690.05 14690.49
62 -+ 61
14692.10
61 -+6z 42 + 41 I 52 - 52 0 00 + 111 (~‘0 = 14098.54 MHz)
14692.64 14096.69” 14097.89” 14098.27” 14098.66”
22-12 ll-tll 32 lx +21 00 --f 10 22
14099.53” 14099.90
22322 11+01
a These transitions were included in the least squares analysis. DThe bracketed transitions could not be well resolved.
EVALUATION
OF COUPLING
CONSTANTS
The spectrum of 1-D-pyrazole was investigated first and most rotational lines were found to have very simple hyperfine structure which could be almost compietely accounted for by using the theory for.‘a single nitrogen nucleus’. The coupling
constants
obtained
% Mol. structttre, 9 (1971) 465-473
from
these transitions
are given
in Table
-2.
QUADRUPOLE
COUPLING
IN l-D-PYRAZOLE-AND
4-D-PYRAZOLE
469
TABLE2 NUCLEAR
QWADRUPOLE
Species
COUPLING
CONSTANTS
%aEw+
Pyrazole++
1-377f0.020
FOR
PYRAZOLE*
MHz)
(in
~bbbW
;dO
;coom
1A41 &0.030
-3.018
-3.961
&to.040
3.167&0_050
0.794
3.24 f0.05
0.88
4-D-Pyrazole
0.82 f0.06
2.28 f0.08
-3.10
-4.12
f0.09
1-D-Pyrazole
0.93
2.14 fO.10
-3-10
-0.40
f0.15
f0.07
XAZ)
X**(2)
-0.35
f0.05
* The quoted errors are estimated from the fit of the spectrum. t The three-coordinated nitrogen is N(l), the two-coordinated is N(2). $ From ref. 4.
VO = 12290.98
-l-5 Fig. 2. Observed
-M
I
-ok
and computer
simulated
I lo
6s
VO
multiplet
for the 3,,
MHz
lk
MHz
+- 3,, transition in I-D-pyrazole.
On a few of the lines however, notably the 3,, + 3,, (see Fig. 2) and r2 -+ 221 transitions, effects due to the second quadrupole nucleus were detected. 2 The analysis of these multiplets produced approximate values of the coupling constants for the second nitrogen nucleus (Table 2). The large uncertainties for N(2) render the calculations of principal quadrupole coupling axes and diagonal components rather uncertain. However more precise results were obtained from a similar study of 4-D-pyrazole where the hyperfine splittings due to the presence of the two quadrupole nuclei are quite large and complex (see Figs. 3 and 4). The values of the x’s for I-D-pyrazole in combination with the values of x’s for the parent compound were used to obtain the approximate 000-101 ‘Jo= 13150.21
I
I -1.0
Fig. 3.
.-OS
“0
MHz
05
l-o
l5
2-o
MHz
Observed and computer simulated multiplet for the 000 + loI transition in 4-D-pyrazole. J. Mol. Slrrrcture, 9 (1971) 465-473
0.75
470
G. L. BLACKMAN,
0oo-
F. R. BURDEN,
A. MISHRA
‘11
MHz
V,,= 14098-54
I -242
R. D. BROWN,
4
I
-1.0
-05
I
/
vo
05
,
10
)5
MHz
Fig. 4. Observed and computer simulated multiplet for the Ooo -Z-Z1 1I transition 4-D-pyrazoie. TABLE
3
ROTATIONAL
CONSTANTS,
OF PRINCIPAL
INERTIAL
PRINCIPAL
MOMENTS
OF INERTIA,
INERTIAL
DEFECT AND ANGLE
OF ROTATION
AXES
I-D-Pyrazoie
CD-Pyrazole
A = B = C = I, = I,, = I, = A = a =
A = B = C = I, = Ib = I, = A = 1 =
9455.27 MHz 8859.75 MHz 4572.88 MHz 53.449 Il. AZ 57.042 u. A= 110.516 u. A2 [&-((I,+&,)] = 0.025 u. ii2 59.13”
9566.16 MHz 8617.86 MHz 4532.36 MHz 52.830 u. A2 58.643 u. A2 111.504 u. A2 iI,-(I,+-&,)] = 0.031 u. A2 27.52”
Conversion factor 5.05376 x lo5 MHz u. AZ.
values of the off-diagonal eIements of the quadrupole coupling tensors of the parent compound, and thereby approximate values of the x’s for 4-D-pyrazole. Final values of x’s for 4-D-pyrazole were obtained in the manner outlined previously4_ The inertial data derived from this analysis are given in Table 3 and are in good agreement with those obtained by Nygaard6.
DISCUSSION
The values of the off-diagonal coupling constants x~,,referred to the inertial axes of pyrazole were derived by combining the observed coupling constants for 4-D-pyrazole and for I-D-pyrazole with those of the parent heterocyclic compound. The values, and the resultant principal quadrupole coupling constants of pyrazole, are listed in TabIe 4.
The previously reported NQR data for pyrazoIeg of e”Qq = 3.995 MHz, q = 0.657 would seem to refer to N(2), for which our values are 4.46 MHz and 0.641. This difference between the xZZ values for the free molecule and the solid J. Mol. Srructrcre,9 (1971) 465-473
QUADRUPOLE TABLE
COUPLING
IN
l-D-PYRAZOLE
AND
471
4-D-PYRAZOLE
4
(in MHz)
~FF-DIAG~NALANDPRINCIPALQUADRUPOLECOUPLINGCONSTANTSOFPYRAZOLE TATIONOFPRINCIPALQUADRUPOLE AXESFROMINERTIALAXES*
AND
ANGLES
OF ORIEN-
-0.8OfO.10
0.70f0.10
2.32fO.10
-3.02
40.3”
-2.00f0.10
0.79
3.69&O.iG
-4.48f0.10
14.6O
-0.7610.10
0.74rtO.13
2.28fO.12
-3.02
40.1”
-2.03&-0.20
0.79
3.70f0.15
-4.49f0.15
14.8’.
* Upper results from 4-D-pyrazole,
lower from l-D-pyrazole,
combining with the parent.
state value is about that usually observed and presumably is to be ascribed to lattice effects. A further comparison may be made with the ab initio calculations of field gradients for nitrogen heterocyclics using contracted gaussian bases3. These are summarized in Table 5 together with our CNDO/~principal values. The agreement is poor for the triply co-ordinate nitrogen, especially for the in-plane asymmetry (V=xP = o-52, ~~~~~~= 0.24). This strongly suggests that present theoretical techniques, even at the ab initio level, do not properly treat H attached to N in these heterocycles. The results for N(2) where no hydrogen is attached, are noticeably better. A qualitative discussion and comparison with some molecular orbital calculations (CNDO/~) of nitrogen 2p orbital occupation numbers was presented in our previous paper_ We have also calculated from the CNDO/~data the angles between the field gradient principal axes and the inertial axes of pyrazole. The values obtained, r&(l) = - 13”, $(2) = 14”, in comparison with the experimentally derived values of 40” and 14”, respectively (Fig. 5), show surprisingly good agreement for the pyridine-type TABLE
nitrogen
but poor agreement
for the pyrrole-type
nitrogen.
A
5
COMPARISON
OF
EXPERIMENTAL
IS-p_
AND
CALCULATED
Ab initio --__ (d3
COUPLING
CONSTANTS
(in
MHz)
CNDO12 (C = - 14.0 MH$ WI*
NW
xxx %YY x-_=
0.72 2.30 -3.02
1.88 3.05 -4.93
1.32 2.15 -3.47
3.69 5.23 -8.92
N(2)
xxx xx9 %=I
0.79 3.69 -4.48
1.31 5.03 -6.33
0.92 3.54 (-4.48)
-0.06 4.91 -4.85
* Values of column (a) multiplied by 0.704 [which yields the experimental value for &N(2)] which corresponds to a nuclear quadrupole moment of 1.13 X 10Sz6 cm2 for N. J. Mol.
Structure,
9 (1971) 465473
G. L. BLACKMAN,
472
R. D. BROWN,
F. R. BURDEN,
A. MISHRA
H(1) Fig. 5. Directions of principal field gradient axes for the nitrogen atoms in pyrazoie. (Dotted line at N(2) indicates bisector of ring angle.)
similar pattern emerges for the calculated angles derived from the ab initio cal-
culations3. Again this points to the need for improved theoretical treatments of five-membered ring nitrogen heterocycles. At a more elementary level, we note (Fig. 5) that the -u-axis for N( 1) deviates about 26” from the NH bond direction while the z-axis at N(2) which could be regarded as indicating the direction of maximum probability for the lone-pair
a-electrons on N(2) deviates 15” from the external bisector of the ring angle. This unsymmetrical orientation of the lone pair electrons might be attributed to the electrostatic effect of the adjacent H atoms, the one on N carrying a notably more positive charge thah. that on C.
ACKNOWLEDGEMENTS
The authors wish to thank Dr. Lise Nygaard for some unpublished results and B. E. .Boulton who prepared the. sample of 4-D-pyrazole. The work was supported by a grant from the Australian Research Grants Committee. % idol. hmure,
9 (1971) 46-73
QUADRUPOLE
COUPLING
IN
1-D-PYRAZOLE
AND
‘t-D-PYRAZOLE
473
REFERENCES
1 E. A. C. LUCKEN, Nuclear Qunrirupole COU~&I~ Cunstunrs, Academic Press, New Y&k, 2 3 4 5
6 7 8 9
1969, Chap. 11. D. W. DAVIFS AND W. C. MACKRODT, Chem. Cornmun., (1967) 1226. E. KOCHANSKI, J. M. LEHN AND B. LEVY, Chem. Phys. Left., 4 (1969) 75. G. L. BLACKMAN, R. D. BROWN AND F. R. BURDEN, J. Mol. Specrrusc., 36 (1971) 528.. D. J. MILLENAND J.R.MoRToN,J. Chem.S0~.,(1960)1523. L. NYGAARD et al., private communciation. E. CHUNG WV AND J. D. VAUGHAN, J. Org. Chem., 35 (1970) 1146. C. H. TOWNES AND A. L. SCHAWLOW, Microwave Spectroscopy, McGraw-Hill, New York, 1955. L. GUIBE AND E. A. C. LUCICEN, Mol. Phys., 14 (lS68) 73. J. Mol.
Structure,
9 (1971) 465-473
Elsevier Publishing
QUADRUPOLE PRINCIPAL
.Company, Amsterdam.
COUPLING QUADRUPOLE
G. L. BLACKMAN.
R. D. BROWN,
465
Printed in the Netherlands
IN l-D-PYRAZOLE COUPLING
F. R. BURDEN
AND 4-D-PYRAZOLE
CONSTANTS
AND
FOR PYRAZOLE
A. MISHRA
Department of Chemistry, Monash Uniuersity, Clayton, Victoria (Australia)
Pth,1970)
ABSTRACT
The hyperfine structure of a number of lines in the microwave spectra of 1-D-pyrazole and 4-D-pyrazole has been measured and analysed, First order quadrupole coupling theory for the two nitrogen atoms has been used to generate computer simulated multiplets for optimal fitting to the corresponding spectrometer tracings. Fittings to all multiplets have been achieved to within the observational limits. By combining the derived coupling constants with those obtained previously for pyrazole itself and assuming that the field gradient tensors are unaltered by isotopic substitution, two independent and closely concordant sets of values have been obtained for the full field gradient tensors at each of the two nitrogens. The results have been compared with field gradient tensors calculated from molecular orbital wave functions. Reasons for the unsatisfactory performance of the CNDO/~ wavefunction are advanced and some implications of the present experimental values for the electronic structure of pyrazole are mentioned.
INTRODUCTION
There have been many attempts to use experimental quadrupole coupling constants for nitrogen nuclei in heterocyclic compounds to obtain greater insight into the electronic structures of such molecules’- ‘. However, for a number of interesting cases the relevant coupling constants, referred to principal field-gradient axes, have hitherto been available only from NQR observations on solids, where the contribution of lattice effects is appreciable and uncertain. The present work was aimed at obtaining the relevant off-diagonal coupling constants in the inertialaxis system of pyrazole so that our previously reported coupling constants foi the gas phase4 could be transformed to the principal-quadrupole axis system. J. Mol. Structure, 9 (1971) 465-473
466
G. L. BLACKMAN,
H (31
Fig. 1. Direction those of pyrazole
R. D. BROWN,
F; R. BURDEN,
A. MISHRA
b
of inertial axes of I-D-pyrazole itself.
(a”, 6”) and 4-D-pyrazole
(a’, 6’) relative to
As the observed hyperfine structure of microwave lines of nitrogen heterocycles can be completely accounted for by a first-order quadrupole coupling theory (i.e. second order effects are too small to be measured so far) the off-diagonal elements must be found by an indirect method. The method previously used’ involves the determination of coupling constants, referred to inertial axes, for two isotopic species such that the direction of inertial axes relative to the nuclei differ appreciably in the two species. We report measurements on I-D- and 4-D-pyrazole, for which the angles of rotation of the inertial axes relative to those in pyrazole itself have been obtained6. They are indicated in Fig. 1.
EXPERIMENTAL
The microwave spectrometer used for this work has been described previousIy4. The needle-shaped, colourless crystals of I-D-pyrazole were collected by J. Mol.
Strucfwe,
9 (1971)
465-473
QuADRUPOLE
COUPLING
IN
I-D-PYRAZOLE
AND
4-D-PYRAZOLE
467
recrystallization from a solution of pyrazole in heavy water. 4-D-pyrazole .was prepared by the method described by Chung and Vaughan’. The rotational constants for both I-D- and 4-D-pyrazoles together with the angles of rotation of. the principal inertial axes on deuteration have been obtained by Nygaard6. All spectra were taken at low pressures and dry-ice temperature in order to achieve maximum resolution. Table 1 lists a11 measured lines for the two deuterospecies of pyrazole. TABLE
1
OBSERVED
AND
CALCULATED
HYPERFINE
TRANSITION
FREQUENCIES
Rotational
Hype&e
Frequency
transition
transition (FI_ --+ F’I,12)
Observed
22-+ 12
13432. I 1
(A)
(MHZ) Calculated
Dtfference
13432.12
0.01 b b b b b b
I-D-Pyrazole 0 00 -+ 101 (v,, = 13432.60 MHz)
11+2, 22 - 32 00--f lo I 1x+01 2L + 22 22+ 11 1
000 -+ 111
(vg = 14028.14 MHz)
22 -
13432.58
13432.82
14027.06
12
11-21 22 +
14028.04
32 1
OO-fll 11+01 22
+
14028.63
22 I
22’10
13432.48 13432.57 13432.59 13432.74 13432.83 13432.92
14027.06 14027.97 14028.05 14028.08 14028.59 14028.67 14028.74
0.00 b b b b
14646.02 14646.13 14646.28 14647.49 14647.71 14648.17
b b b 0.01 0.01 0.01
12290.24 12290.29 12290.41 12291.12 12291.16 12291.32 12291.48 12291.62
b b 0.01 b b
212 + 221 ho =
14647.18 MHz)
ll-tll
14646.13
32 -+ 32
312 -+ 321 (v. = 12290.98 MHz)
22
-+
22
42
+
42
31+
31
2x4
2,
21
21
+
1
42 -+ 4t I 32 32 31 -+ 30 1 52 52 I 41 + 41 30 --f 31 22 -+ 22
14647.38 14647.70 14648.16
12290.29 12290.40 12291.17 12291.30 12291.48 12291.58
J. Mol.
Structure,
b”
0.02 0.00 0.04
9 (1971) 465473
468
G. L. BLACKMAN,
TABLE
R. D. BROWN,
F. R. BURDEN,
A: MISHRA
1 (continued)
Rotational transition
Hyper-ne transition (FIZZ + F’I,,,)
Frequency (MHz) nbserrted
Caicrdated
Diference
13149.03 13149.19 13149.38 13150.30 13150.38 13150.57 13152.29
0.00 0.00
0.00 b’ b 0.01 0.01
16608.20 16608.59 16608,69 16608.82 16609.18 16609.28 16610.32 16610.67 16610.81
0.02 b b b -0.01 b -0.01 0.00 b
14689.62 14689.64 14690.01 14690.09 14690.14 14690.49 14692.10 14692.19 14692.64
b b b b b 0.00 b b 0.00
14096.69 14097.88 14098.26 14098.66 14099.40 14099.52 14099.92
0.00 -0.01 -0.01 0.00 b -0.01 0.02
(B) 4-D-PyrazoIe 0
-
lo1
13150.21 MHz)
(i," =
13149.03’ 13149.19’ 13149.38=
22-+10 22
+-22
ll-+Ol
13150.36
oo+-11 22
-32
1
22'12
13150.56* 13152.28”
h--+22
16608.185
31 + 31 41 41 12 + + 12 I
16608.65
11+&
322 (v,,
--;’ 331 = 16609.43 MHz)
52
16609.19”
+52
30 + 32 + 41 21 -
30 32 42 21
16610.33” 16610.67”
Ll --f 550
(v, = 14690.80 MHz)
32 -+ 32 51 -+ 5‘ I 50 -+- 50 41-+4t 72 72 I
14689.63 14690.05 14690.49
62 -+ 61
14692.10
61 -+6z 42 + 41 I 52 - 52 0 00 + 111 (~‘0 = 14098.54 MHz)
14692.64 14096.69” 14097.89” 14098.27” 14098.66”
22-12 ll-tll 32 lx +21 00 --f 10 22
14099.53” 14099.90
22322 11+01
a These transitions were included in the least squares analysis. DThe bracketed transitions could not be well resolved.
EVALUATION
OF COUPLING
CONSTANTS
The spectrum of 1-D-pyrazole was investigated first and most rotational lines were found to have very simple hyperfine structure which could be almost compietely accounted for by using the theory for.‘a single nitrogen nucleus’. The coupling
constants
obtained
% Mol. structttre, 9 (1971) 465-473
from
these transitions
are given
in Table
-2.
QUADRUPOLE
COUPLING
IN l-D-PYRAZOLE-AND
4-D-PYRAZOLE
469
TABLE2 NUCLEAR
QWADRUPOLE
Species
COUPLING
CONSTANTS
%aEw+
Pyrazole++
1-377f0.020
FOR
PYRAZOLE*
MHz)
(in
~bbbW
;dO
;coom
1A41 &0.030
-3.018
-3.961
&to.040
3.167&0_050
0.794
3.24 f0.05
0.88
4-D-Pyrazole
0.82 f0.06
2.28 f0.08
-3.10
-4.12
f0.09
1-D-Pyrazole
0.93
2.14 fO.10
-3-10
-0.40
f0.15
f0.07
XAZ)
X**(2)
-0.35
f0.05
* The quoted errors are estimated from the fit of the spectrum. t The three-coordinated nitrogen is N(l), the two-coordinated is N(2). $ From ref. 4.
VO = 12290.98
-l-5 Fig. 2. Observed
-M
I
-ok
and computer
simulated
I lo
6s
VO
multiplet
for the 3,,
MHz
lk
MHz
+- 3,, transition in I-D-pyrazole.
On a few of the lines however, notably the 3,, + 3,, (see Fig. 2) and r2 -+ 221 transitions, effects due to the second quadrupole nucleus were detected. 2 The analysis of these multiplets produced approximate values of the coupling constants for the second nitrogen nucleus (Table 2). The large uncertainties for N(2) render the calculations of principal quadrupole coupling axes and diagonal components rather uncertain. However more precise results were obtained from a similar study of 4-D-pyrazole where the hyperfine splittings due to the presence of the two quadrupole nuclei are quite large and complex (see Figs. 3 and 4). The values of the x’s for I-D-pyrazole in combination with the values of x’s for the parent compound were used to obtain the approximate 000-101 ‘Jo= 13150.21
I
I -1.0
Fig. 3.
.-OS
“0
MHz
05
l-o
l5
2-o
MHz
Observed and computer simulated multiplet for the 000 + loI transition in 4-D-pyrazole. J. Mol. Slrrrcture, 9 (1971) 465-473
0.75
470
G. L. BLACKMAN,
0oo-
F. R. BURDEN,
A. MISHRA
‘11
MHz
V,,= 14098-54
I -242
R. D. BROWN,
4
I
-1.0
-05
I
/
vo
05
,
10
)5
MHz
Fig. 4. Observed and computer simulated multiplet for the Ooo -Z-Z1 1I transition 4-D-pyrazoie. TABLE
3
ROTATIONAL
CONSTANTS,
OF PRINCIPAL
INERTIAL
PRINCIPAL
MOMENTS
OF INERTIA,
INERTIAL
DEFECT AND ANGLE
OF ROTATION
AXES
I-D-Pyrazoie
CD-Pyrazole
A = B = C = I, = I,, = I, = A = a =
A = B = C = I, = Ib = I, = A = 1 =
9455.27 MHz 8859.75 MHz 4572.88 MHz 53.449 Il. AZ 57.042 u. A= 110.516 u. A2 [&-((I,+&,)] = 0.025 u. ii2 59.13”
9566.16 MHz 8617.86 MHz 4532.36 MHz 52.830 u. A2 58.643 u. A2 111.504 u. A2 iI,-(I,+-&,)] = 0.031 u. A2 27.52”
Conversion factor 5.05376 x lo5 MHz u. AZ.
values of the off-diagonal eIements of the quadrupole coupling tensors of the parent compound, and thereby approximate values of the x’s for 4-D-pyrazole. Final values of x’s for 4-D-pyrazole were obtained in the manner outlined previously4_ The inertial data derived from this analysis are given in Table 3 and are in good agreement with those obtained by Nygaard6.
DISCUSSION
The values of the off-diagonal coupling constants x~,,referred to the inertial axes of pyrazole were derived by combining the observed coupling constants for 4-D-pyrazole and for I-D-pyrazole with those of the parent heterocyclic compound. The values, and the resultant principal quadrupole coupling constants of pyrazole, are listed in TabIe 4.
The previously reported NQR data for pyrazoIeg of e”Qq = 3.995 MHz, q = 0.657 would seem to refer to N(2), for which our values are 4.46 MHz and 0.641. This difference between the xZZ values for the free molecule and the solid J. Mol. Srructrcre,9 (1971) 465-473
QUADRUPOLE TABLE
COUPLING
IN
l-D-PYRAZOLE
AND
471
4-D-PYRAZOLE
4
(in MHz)
~FF-DIAG~NALANDPRINCIPALQUADRUPOLECOUPLINGCONSTANTSOFPYRAZOLE TATIONOFPRINCIPALQUADRUPOLE AXESFROMINERTIALAXES*
AND
ANGLES
OF ORIEN-
-0.8OfO.10
0.70f0.10
2.32fO.10
-3.02
40.3”
-2.00f0.10
0.79
3.69&O.iG
-4.48f0.10
14.6O
-0.7610.10
0.74rtO.13
2.28fO.12
-3.02
40.1”
-2.03&-0.20
0.79
3.70f0.15
-4.49f0.15
14.8’.
* Upper results from 4-D-pyrazole,
lower from l-D-pyrazole,
combining with the parent.
state value is about that usually observed and presumably is to be ascribed to lattice effects. A further comparison may be made with the ab initio calculations of field gradients for nitrogen heterocyclics using contracted gaussian bases3. These are summarized in Table 5 together with our CNDO/~principal values. The agreement is poor for the triply co-ordinate nitrogen, especially for the in-plane asymmetry (V=xP = o-52, ~~~~~~= 0.24). This strongly suggests that present theoretical techniques, even at the ab initio level, do not properly treat H attached to N in these heterocycles. The results for N(2) where no hydrogen is attached, are noticeably better. A qualitative discussion and comparison with some molecular orbital calculations (CNDO/~) of nitrogen 2p orbital occupation numbers was presented in our previous paper_ We have also calculated from the CNDO/~data the angles between the field gradient principal axes and the inertial axes of pyrazole. The values obtained, r&(l) = - 13”, $(2) = 14”, in comparison with the experimentally derived values of 40” and 14”, respectively (Fig. 5), show surprisingly good agreement for the pyridine-type TABLE
nitrogen
but poor agreement
for the pyrrole-type
nitrogen.
A
5
COMPARISON
OF
EXPERIMENTAL
IS-p_
AND
CALCULATED
Ab initio --__ (d3
COUPLING
CONSTANTS
(in
MHz)
CNDO12 (C = - 14.0 MH$ WI*
NW
xxx %YY x-_=
0.72 2.30 -3.02
1.88 3.05 -4.93
1.32 2.15 -3.47
3.69 5.23 -8.92
N(2)
xxx xx9 %=I
0.79 3.69 -4.48
1.31 5.03 -6.33
0.92 3.54 (-4.48)
-0.06 4.91 -4.85
* Values of column (a) multiplied by 0.704 [which yields the experimental value for &N(2)] which corresponds to a nuclear quadrupole moment of 1.13 X 10Sz6 cm2 for N. J. Mol.
Structure,
9 (1971) 465473
G. L. BLACKMAN,
472
R. D. BROWN,
F. R. BURDEN,
A. MISHRA
H(1) Fig. 5. Directions of principal field gradient axes for the nitrogen atoms in pyrazoie. (Dotted line at N(2) indicates bisector of ring angle.)
similar pattern emerges for the calculated angles derived from the ab initio cal-
culations3. Again this points to the need for improved theoretical treatments of five-membered ring nitrogen heterocycles. At a more elementary level, we note (Fig. 5) that the -u-axis for N( 1) deviates about 26” from the NH bond direction while the z-axis at N(2) which could be regarded as indicating the direction of maximum probability for the lone-pair
a-electrons on N(2) deviates 15” from the external bisector of the ring angle. This unsymmetrical orientation of the lone pair electrons might be attributed to the electrostatic effect of the adjacent H atoms, the one on N carrying a notably more positive charge thah. that on C.
ACKNOWLEDGEMENTS
The authors wish to thank Dr. Lise Nygaard for some unpublished results and B. E. .Boulton who prepared the. sample of 4-D-pyrazole. The work was supported by a grant from the Australian Research Grants Committee. % idol. hmure,
9 (1971) 46-73
QUADRUPOLE
COUPLING
IN
1-D-PYRAZOLE
AND
‘t-D-PYRAZOLE
473
REFERENCES
1 E. A. C. LUCKEN, Nuclear Qunrirupole COU~&I~ Cunstunrs, Academic Press, New Y&k, 2 3 4 5
6 7 8 9
1969, Chap. 11. D. W. DAVIFS AND W. C. MACKRODT, Chem. Cornmun., (1967) 1226. E. KOCHANSKI, J. M. LEHN AND B. LEVY, Chem. Phys. Left., 4 (1969) 75. G. L. BLACKMAN, R. D. BROWN AND F. R. BURDEN, J. Mol. Specrrusc., 36 (1971) 528.. D. J. MILLENAND J.R.MoRToN,J. Chem.S0~.,(1960)1523. L. NYGAARD et al., private communciation. E. CHUNG WV AND J. D. VAUGHAN, J. Org. Chem., 35 (1970) 1146. C. H. TOWNES AND A. L. SCHAWLOW, Microwave Spectroscopy, McGraw-Hill, New York, 1955. L. GUIBE AND E. A. C. LUCICEN, Mol. Phys., 14 (lS68) 73. J. Mol.
Structure,
9 (1971) 465-473