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Br-nuclear quadrupole coupling in the rotational spectrum of 1-bromoadamantane
Journal of Molecular Structure, 245 (1991) 97-102
97
Elsevier Science Publishers B.V., Amsterdam
B r - N U C L E A R Q U A D R U P O L E C O U P L I N G IN T H E R O T A T I O N A L S P E C T R U M OF 1 - B R O M O A D A M A N T A N E
A.C. LEGON, D.J. MILLEN, A.J. STEEL and A.L. WALLWORK
Christopher Ingold Laboratories, Department of Chemistry, University College London, 20 Gordon Street, London WCIH OAJ (United Kingdom) (Received 4 July 1990)
ABSTRACT The nuclear quadrupole coupling constants X(V9Br) and X(81Br) have been determined for the ground state of 1-bromoadamantane by pulsed-nozzle, Fourier-transform microwave spectroscopy. A comparison of similar coupling constants for the series methyl bromide, ethyl bromide, t-butyl bromide and 1-bromoadamantane shows that the ionic character of the C-Br bond increases along this series.
INTRODUCTION
Nuclear electric quadrupole hyperfine structure in rotational spectra provides a route to information about the distribution of electronic charge in the vicinity of the responsible nucleus via the nuclear quadrupole coupling constant )~(X). In an axially symmetric molecule, when X lies on the symmetry axis z, z ( X ) is related to the z-component - 0 2V / O z 2 of the electric field gradient at X by x(X)
= - (eQ/h )O2V/Oz 2
(1)
This equation provides a basis for a comparison of the changes in electric field gradient at a halogen, X, in a series of monohalogenohydrocarbons in which the environment of the C-X bond is systematically varied. In this way, a gradual change in the electronic distribution at C1 along the series methyl chloride, ethyl chloride, t-butyl chloride and 1-chloroadamantane has been detected and discussed recently [ 1 ]. This change was attributed to the increasing ionic character of the C-C1 bond along the series. We report here an experimental investigation of Br-nuclear quadrupole coupling in the rotational spectrum of 1-bromoadamantane. The Br-nuclear quadrupole coupling constant thereby determined allows any trend of electronic change along the series methyl bromide, ethyl bromide, t-butyl bromide and 0022-2860/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
98 1-bromoadamantane to be discerned and then to be compared with that for the corresponding series of chloro compounds.
EXPERIMENTAL
The ground-state rotational spectra of the two isotopomers 79Br- and SlBrbromoadamantane were recorded and analysed by using a pulsed-nozzle, Fourier-transform microwave spectrometer of the Balle-Flygare type [2,3]. Solid 1-bromoadamantane (Aldrich) in a channel surrounding the orifice of a General Valve Corp. solenoid valve ( P / N 8-14-900) was heated to a temperature of ~ 330 K. The vapour pressure of 1-bromoadamantane at this temperature was sufficient that when the vapour was entrained in argon at a pressure of 0.3 atm and pulsed through the 0.7 mm diameter nozzle into the evacuated Fabry-Pdrot cavity, rotational transitions with good signal-to-noise ratio were readily observed. Br-nuclear hyperfine structure was easily resolved, each component having a full width at half height of ca. 16 kHz (see Fig. 1 ). The accuracy of frequency measurement was ~ 2 kHz.
L
10041.30
,
,
,
.40
,
.50
,
,
.60
.
.
.
.70
.
.80
Frequency/ MHz
Fig. 1. A frequency domain recording of the ( F = 17/2 ~ 15/2, K = 0 ), ( F = 15/2 ~- 13/2, K = 0) and (F=19/2~17/2, K = 2 ) components of the J = 9 ~ - 8 transition in the ground state of [79Br]-1bromoadamantane. The observed frequencies are 10041.4409, 10041.4772 and 10041.6249 MHz, respectively. The stick diagram indicates the calculated frequencies and intensities of these hyperfine components.
99 RESULTS
The ground-state J = 8+-7 and 9,--8 rotational transitions of 79Br- and 81Brbromoadamantane were observed in the frequency range 8-12 GHz. Because 1-bromoadamantane is a symmetric-top molecule each transition was split into several K ~ K components and in turn each of these carried Br-nuclear quadrupole hyperfine structure. Assignments and observed frequencies for the two isotopomers are recorded in Table 1. In an initial analysis of transition frequencies, the rotational constant Bo, the centrifugal distortion constants D j and DjK, and the bromine nuclear quadrupole coupling constant z(Br) were determined by a standard, iterative least-squares fit in which the Hamiltonian [4] H = Bo j2
_ Djj4
_
DjKJ~ j 2 _ 1 / 6 Q : V E
(2)
was constructed in the coupled basis IBr + J = F and diagonalized in blocks of F. The residuals were in fact somewhat larger than expected from the estimated error of frequency measurement. The reason for this was the significant contribution to the hyperfine splitting caused by magnetic coupling of Im and J. Accordingly, diagonal elements of the following term HsR were added to the TABLE 1 Observed and calculated rotational transition
frequencies of
[79Br]- and
[81Br]-l-
bromoadamantane
Transition
[79Br ] - 1-bromoadamantane
[81Br] -1-bromoadamantane
Pob~ (MHz)
~ob.- P~¢ (kHz)
Pob~ (MHz)
/lobs--~'¢~d¢(kHz)
d+ l~J
K
F',---F"
8*-7
0
17/2,-15/2 19/2`-17/2 15/2,-13/2 13/2+-11/2
8923.3834 8923.4141 8926.3049 8926.3618
-- 1.7 --0.7 0.2 --0.6
8828.9160 8828.9410 8831.3818 8831.4229
--2.6 1.0 0.4 0.7
1
19/2`-17/2 17/2,-15/2 13/2,--11/2 15/2,--13/2
8922.8602 8924.2802 8925.3227 8926.8401
--0.6 0.8 1.2 --0.9
8828.4750 8829.6704 8830.5577 8831.8230
--0.2 0.3 0.3 --1.0
2
19/2`-17/2 13/2,-11/2 17/2`-15/2 15/2,-13/2
8921.1933 8922.2145 8926.9741 8928.4390
--0.3 --0.4 --0.1 --0.1
8827.0776 8827.9743 8831.9328 8833.1438
0.5 --0.2 --0.6 --0.2
3
13/2+-11/2 19/2,-17/2 15/2`-13/2 17/2,-15/2
8917.0912 8918.3994 8931.0670 8931.5053
0.7 1.3 --0.1 --1.1
8823.7067 8824.7341 8835.3194 8835.7348
--0.3 --1.0 0.3 0.2
100 TABLE 1 (continued) Transition J+l*-J
9*-8
[79Br] -1-bromoadamantane
[s~Br ]- 1-bromoadamantane
/Jobs (MHz)
Vob~--V¢~¢ (kHz)
Robs (MHz)
Vob~--V¢~¢(kHz)
K
F',---F"
4
13/2.-11/2 19/2--17/2 15/2.-13/2 17/2.-15/2
8910.0268 8914.4487 8934.6720 8937.9386
0.8 0.4 -0.8 0.1
8817.8090 8821.4319 8838.3129 8841.1179
-0.2 1.4 0.2 0.1
5
13/2.-11/2 19/2.-17/2 15/2.-13/2 17/2.-15/2
8901.1260 8909.3078 8939.1842 8946.3614
1.1 0.9 -0.4 0.0
8810.3544 8817.1378 8842.0738 8848.1483
-0.2 0.5 -0.7 1.3
6
19/2.-17/2 17/2.-15/2
8902.9251 8956.8966
0.9 -0.5
0
19/2.-17/2 21/2.-19/2 17/2,--15/2 15/2.-13/2
10039.1506 10039.1839 10041.4409 10041.4772
-5.7 5.5 1.6 0.0
9932.8222 9932.8452 9934.7493 9934.7805
-3.8 3.2 -2.3 2.3
1
17/2.-15/2 21/2.-19/2 19/2.-17/2 15/2.-13/2
10041.8343 10038.7770 10039.7719 10040.7758
-0.2 -0.4 -0.3 0.2
9935.0787 9932.5062 9933.3433 9934.1956
0.4 0.5 0.0 0.6
2
19/2.-17/2 21/2.-19/2 15/2.-13/2 17/2.-15/2
10041.6249 10037.5716 10038.6789 10043.0150
-0.7 -0.2 0.6 0.1
9934.8996 9931.4945 9932.4505 9936.0535
0.3 -0.3 0.2 -1.3
3
15/2.-13/2 21/2.-19/2 19/2.-17/2 17/2~-15/2
10035.2068 10035.5533 10044.7361 10044.9651
-0.4 -0.3 1.9 0.4
9929.5604 9929.8045 9937.5064 9937.6697
0.7 0.7 -0.3 -0.2
4
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10030.3962 10032.7096 10047.6590 10049.1288
-2.1 0.1 1.2 0.5
9925.5481 9927.4239 9939.9059 9941.1859
-0.2 0.7 0.5 -0.8
5
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10024.3012 10029.0189 10051.0588 10054.8509
0.9 - 1.4 0.5 0.2
9920.4517 9924.3396 9942.7363 9945.9689
1.2 0.1 0.3 -0.8
6
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10016.9729 10024.4588 10055.1211 10061.9595
-0.6 -2.2 -0.2 0.9
9920.5332 9951.8957
- 1.7 -0.1
101 TABLE 2 Observed ground-state spectroscopic constants of [79Br ] - and [81Br ] - 1-bromoadamantane Spectroscopic constant
[ 79Br ] - 1bromoadamantane
[SlBr ] - 1 bromoadamantane
Bo (MHz) D j (kHz) DjK (kHz) x(Br) (MHz) CN (kHz) CK (kHz)
557.7828(1) 0.010(1) 0.034 (3) 503.524(9) 1.81(3) 2.4(5)
551.8673(1) 0.009(1) 0.033 (4) 420.646(9) 1.60(3) 1.3(6)
H matrix [4] HsR=ejfl.J
(3)
where for a symmetric-top molecule the coefficient Cj, i becomes Cj, i = Cg -- ( CK -- C N ) K 2 / J ( J + 1 )
(4)
in which CN and CK are the perpendicular and axial spin-rotation coupling constants, respectively. Such a first-order treatment of the spin-rotation coupling was satisfactory and reduced the residuals of the fit to those given in Table 1. Except for the nearly degenerate pairs of hyperfine components in the K=O transitions, the residuals are as expected from the estimated error of frequency measurement. These particular pairs of components were not completely resolved for either the VgBr or the SlBr isotopomer, hence the larger residuals in each case. The final set of spectroscopic constants is shown in Table 2. The quoted errors in CK and CN are probably underestimates in view of contributions to the observed frequencies that are only a few kHz. DISCUSSION
The values of x(Br) reported here for 1-bromoadamantane complete the series of monobrominated hydrocarbons RBr, where R-- methyl, ethyl, t-butyl and adamantyl. A summary of the x(Br) for this series is given in Table 3. A recent paper has discussed the chlorine nuclear quadrupole coupling constants x(C1) in the corresponding monochlorinated series [1 ]. We note from Table 3 that the magnitude ofx(79Br) decreases in the order methyl [5] > ethyl [6] > t-butyl [7] > adamantyl. This trend parallels that for the series RC1 [1]. The ionic character ic of the C-Br bond can be determined from the X(Br) with the aid of the equation put forward by Townes and Dailey [8] ic = 1 + hx(Br)/eQq41o (Br)
(5)
where (eQq41o/h)= -796.76 MHz is the quadrupole coupling constant for a
102 TABLE 3 Comparison of X(79Br) and the calculated ionic character of the C-Br bond in some monobrominated hydrocarbons
Methyl Ethyl t-Butyl Adamantyl
X(79Br) (MHz)
Ionic character of C-Br bond (%)
577.143 (8) a 541.7535 513.7 (8)c 503.524 (9)d
25.0 29.6 33.3 (2) 34.6
aRef. 5.5Value of Xzz(79Br) along the CBr bond direction (z) calculated from ~ , Xbb,Xab given in ref. 6. CRef. 7. dThis work.
single valence p-electron on bromine 79Br. From this simple formula and the )c(Br) values, we find that the ionic contribution to the C-Br bond in 1-bromoadamantane is slightly greater than in t-butyl bromide but significantly more so than in methyl and ethyl bromides, as shown in Table 3. We cannot comment on whether the similar but slightly larger ionic character in 1-bromoadamantane is paralleled by a flattening of the bridgehead position to which Br is attached and lengthening of the C-Br bond, as established in t-butyl bromide [7 ], because an rs-structure of 1-bromoadamantane is not yet available. ACKNOWLEDGEMENT
We thank the SERC for the award of a research studentship to A.L.W.
REFERENCES 1 2 3 4 5 6 7 8
M.C. Ellis, A.C. Legon, C.A. Rego and D.J. MiUen, J. Mol. Struct., 200 (1989) 353. T.J. Balle and W.H. Flygare, Rev. Sci. Instrum., 52 (1981) 35. A.C. Legon, Annu. Rev. Phys. Chem., 34 (1983) 275. W. Gordy and R.L. Cook, in A. Weissberger (Ed.), Microwave Molecular Spectra, Techniques of Organic Chemistry, Vol. IX, Interscience, 1970. E. Arimondo, J.G. Baker, P. Glorieux, T. Oka and J. Sakai, J. Mol. Spectrosc., 82 (1980) 54. J. Gripp, H. Dreizler and R. Schwarz, Z. Naturforsch. Teil A, 40 (1985) 575. A.C. Legon, D.J. MiUen and A. Samsam-Baktiari, J. Mol. Struct., 52 (1979) 71. C.H. Townes and B.P. Dailey, J. Chem. Phys., 17 (1949) 782.
97
Elsevier Science Publishers B.V., Amsterdam
B r - N U C L E A R Q U A D R U P O L E C O U P L I N G IN T H E R O T A T I O N A L S P E C T R U M OF 1 - B R O M O A D A M A N T A N E
A.C. LEGON, D.J. MILLEN, A.J. STEEL and A.L. WALLWORK
Christopher Ingold Laboratories, Department of Chemistry, University College London, 20 Gordon Street, London WCIH OAJ (United Kingdom) (Received 4 July 1990)
ABSTRACT The nuclear quadrupole coupling constants X(V9Br) and X(81Br) have been determined for the ground state of 1-bromoadamantane by pulsed-nozzle, Fourier-transform microwave spectroscopy. A comparison of similar coupling constants for the series methyl bromide, ethyl bromide, t-butyl bromide and 1-bromoadamantane shows that the ionic character of the C-Br bond increases along this series.
INTRODUCTION
Nuclear electric quadrupole hyperfine structure in rotational spectra provides a route to information about the distribution of electronic charge in the vicinity of the responsible nucleus via the nuclear quadrupole coupling constant )~(X). In an axially symmetric molecule, when X lies on the symmetry axis z, z ( X ) is related to the z-component - 0 2V / O z 2 of the electric field gradient at X by x(X)
= - (eQ/h )O2V/Oz 2
(1)
This equation provides a basis for a comparison of the changes in electric field gradient at a halogen, X, in a series of monohalogenohydrocarbons in which the environment of the C-X bond is systematically varied. In this way, a gradual change in the electronic distribution at C1 along the series methyl chloride, ethyl chloride, t-butyl chloride and 1-chloroadamantane has been detected and discussed recently [ 1 ]. This change was attributed to the increasing ionic character of the C-C1 bond along the series. We report here an experimental investigation of Br-nuclear quadrupole coupling in the rotational spectrum of 1-bromoadamantane. The Br-nuclear quadrupole coupling constant thereby determined allows any trend of electronic change along the series methyl bromide, ethyl bromide, t-butyl bromide and 0022-2860/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
98 1-bromoadamantane to be discerned and then to be compared with that for the corresponding series of chloro compounds.
EXPERIMENTAL
The ground-state rotational spectra of the two isotopomers 79Br- and SlBrbromoadamantane were recorded and analysed by using a pulsed-nozzle, Fourier-transform microwave spectrometer of the Balle-Flygare type [2,3]. Solid 1-bromoadamantane (Aldrich) in a channel surrounding the orifice of a General Valve Corp. solenoid valve ( P / N 8-14-900) was heated to a temperature of ~ 330 K. The vapour pressure of 1-bromoadamantane at this temperature was sufficient that when the vapour was entrained in argon at a pressure of 0.3 atm and pulsed through the 0.7 mm diameter nozzle into the evacuated Fabry-Pdrot cavity, rotational transitions with good signal-to-noise ratio were readily observed. Br-nuclear hyperfine structure was easily resolved, each component having a full width at half height of ca. 16 kHz (see Fig. 1 ). The accuracy of frequency measurement was ~ 2 kHz.
L
10041.30
,
,
,
.40
,
.50
,
,
.60
.
.
.
.70
.
.80
Frequency/ MHz
Fig. 1. A frequency domain recording of the ( F = 17/2 ~ 15/2, K = 0 ), ( F = 15/2 ~- 13/2, K = 0) and (F=19/2~17/2, K = 2 ) components of the J = 9 ~ - 8 transition in the ground state of [79Br]-1bromoadamantane. The observed frequencies are 10041.4409, 10041.4772 and 10041.6249 MHz, respectively. The stick diagram indicates the calculated frequencies and intensities of these hyperfine components.
99 RESULTS
The ground-state J = 8+-7 and 9,--8 rotational transitions of 79Br- and 81Brbromoadamantane were observed in the frequency range 8-12 GHz. Because 1-bromoadamantane is a symmetric-top molecule each transition was split into several K ~ K components and in turn each of these carried Br-nuclear quadrupole hyperfine structure. Assignments and observed frequencies for the two isotopomers are recorded in Table 1. In an initial analysis of transition frequencies, the rotational constant Bo, the centrifugal distortion constants D j and DjK, and the bromine nuclear quadrupole coupling constant z(Br) were determined by a standard, iterative least-squares fit in which the Hamiltonian [4] H = Bo j2
_ Djj4
_
DjKJ~ j 2 _ 1 / 6 Q : V E
(2)
was constructed in the coupled basis IBr + J = F and diagonalized in blocks of F. The residuals were in fact somewhat larger than expected from the estimated error of frequency measurement. The reason for this was the significant contribution to the hyperfine splitting caused by magnetic coupling of Im and J. Accordingly, diagonal elements of the following term HsR were added to the TABLE 1 Observed and calculated rotational transition
frequencies of
[79Br]- and
[81Br]-l-
bromoadamantane
Transition
[79Br ] - 1-bromoadamantane
[81Br] -1-bromoadamantane
Pob~ (MHz)
~ob.- P~¢ (kHz)
Pob~ (MHz)
/lobs--~'¢~d¢(kHz)
d+ l~J
K
F',---F"
8*-7
0
17/2,-15/2 19/2`-17/2 15/2,-13/2 13/2+-11/2
8923.3834 8923.4141 8926.3049 8926.3618
-- 1.7 --0.7 0.2 --0.6
8828.9160 8828.9410 8831.3818 8831.4229
--2.6 1.0 0.4 0.7
1
19/2`-17/2 17/2,-15/2 13/2,--11/2 15/2,--13/2
8922.8602 8924.2802 8925.3227 8926.8401
--0.6 0.8 1.2 --0.9
8828.4750 8829.6704 8830.5577 8831.8230
--0.2 0.3 0.3 --1.0
2
19/2`-17/2 13/2,-11/2 17/2`-15/2 15/2,-13/2
8921.1933 8922.2145 8926.9741 8928.4390
--0.3 --0.4 --0.1 --0.1
8827.0776 8827.9743 8831.9328 8833.1438
0.5 --0.2 --0.6 --0.2
3
13/2+-11/2 19/2,-17/2 15/2`-13/2 17/2,-15/2
8917.0912 8918.3994 8931.0670 8931.5053
0.7 1.3 --0.1 --1.1
8823.7067 8824.7341 8835.3194 8835.7348
--0.3 --1.0 0.3 0.2
100 TABLE 1 (continued) Transition J+l*-J
9*-8
[79Br] -1-bromoadamantane
[s~Br ]- 1-bromoadamantane
/Jobs (MHz)
Vob~--V¢~¢ (kHz)
Robs (MHz)
Vob~--V¢~¢(kHz)
K
F',---F"
4
13/2.-11/2 19/2--17/2 15/2.-13/2 17/2.-15/2
8910.0268 8914.4487 8934.6720 8937.9386
0.8 0.4 -0.8 0.1
8817.8090 8821.4319 8838.3129 8841.1179
-0.2 1.4 0.2 0.1
5
13/2.-11/2 19/2.-17/2 15/2.-13/2 17/2.-15/2
8901.1260 8909.3078 8939.1842 8946.3614
1.1 0.9 -0.4 0.0
8810.3544 8817.1378 8842.0738 8848.1483
-0.2 0.5 -0.7 1.3
6
19/2.-17/2 17/2.-15/2
8902.9251 8956.8966
0.9 -0.5
0
19/2.-17/2 21/2.-19/2 17/2,--15/2 15/2.-13/2
10039.1506 10039.1839 10041.4409 10041.4772
-5.7 5.5 1.6 0.0
9932.8222 9932.8452 9934.7493 9934.7805
-3.8 3.2 -2.3 2.3
1
17/2.-15/2 21/2.-19/2 19/2.-17/2 15/2.-13/2
10041.8343 10038.7770 10039.7719 10040.7758
-0.2 -0.4 -0.3 0.2
9935.0787 9932.5062 9933.3433 9934.1956
0.4 0.5 0.0 0.6
2
19/2.-17/2 21/2.-19/2 15/2.-13/2 17/2.-15/2
10041.6249 10037.5716 10038.6789 10043.0150
-0.7 -0.2 0.6 0.1
9934.8996 9931.4945 9932.4505 9936.0535
0.3 -0.3 0.2 -1.3
3
15/2.-13/2 21/2.-19/2 19/2.-17/2 17/2~-15/2
10035.2068 10035.5533 10044.7361 10044.9651
-0.4 -0.3 1.9 0.4
9929.5604 9929.8045 9937.5064 9937.6697
0.7 0.7 -0.3 -0.2
4
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10030.3962 10032.7096 10047.6590 10049.1288
-2.1 0.1 1.2 0.5
9925.5481 9927.4239 9939.9059 9941.1859
-0.2 0.7 0.5 -0.8
5
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10024.3012 10029.0189 10051.0588 10054.8509
0.9 - 1.4 0.5 0.2
9920.4517 9924.3396 9942.7363 9945.9689
1.2 0.1 0.3 -0.8
6
15/2.-13/2 21/2.-19/2 17/2.-15/2 19/2.-17/2
10016.9729 10024.4588 10055.1211 10061.9595
-0.6 -2.2 -0.2 0.9
9920.5332 9951.8957
- 1.7 -0.1
101 TABLE 2 Observed ground-state spectroscopic constants of [79Br ] - and [81Br ] - 1-bromoadamantane Spectroscopic constant
[ 79Br ] - 1bromoadamantane
[SlBr ] - 1 bromoadamantane
Bo (MHz) D j (kHz) DjK (kHz) x(Br) (MHz) CN (kHz) CK (kHz)
557.7828(1) 0.010(1) 0.034 (3) 503.524(9) 1.81(3) 2.4(5)
551.8673(1) 0.009(1) 0.033 (4) 420.646(9) 1.60(3) 1.3(6)
H matrix [4] HsR=ejfl.J
(3)
where for a symmetric-top molecule the coefficient Cj, i becomes Cj, i = Cg -- ( CK -- C N ) K 2 / J ( J + 1 )
(4)
in which CN and CK are the perpendicular and axial spin-rotation coupling constants, respectively. Such a first-order treatment of the spin-rotation coupling was satisfactory and reduced the residuals of the fit to those given in Table 1. Except for the nearly degenerate pairs of hyperfine components in the K=O transitions, the residuals are as expected from the estimated error of frequency measurement. These particular pairs of components were not completely resolved for either the VgBr or the SlBr isotopomer, hence the larger residuals in each case. The final set of spectroscopic constants is shown in Table 2. The quoted errors in CK and CN are probably underestimates in view of contributions to the observed frequencies that are only a few kHz. DISCUSSION
The values of x(Br) reported here for 1-bromoadamantane complete the series of monobrominated hydrocarbons RBr, where R-- methyl, ethyl, t-butyl and adamantyl. A summary of the x(Br) for this series is given in Table 3. A recent paper has discussed the chlorine nuclear quadrupole coupling constants x(C1) in the corresponding monochlorinated series [1 ]. We note from Table 3 that the magnitude ofx(79Br) decreases in the order methyl [5] > ethyl [6] > t-butyl [7] > adamantyl. This trend parallels that for the series RC1 [1]. The ionic character ic of the C-Br bond can be determined from the X(Br) with the aid of the equation put forward by Townes and Dailey [8] ic = 1 + hx(Br)/eQq41o (Br)
(5)
where (eQq41o/h)= -796.76 MHz is the quadrupole coupling constant for a
102 TABLE 3 Comparison of X(79Br) and the calculated ionic character of the C-Br bond in some monobrominated hydrocarbons
Methyl Ethyl t-Butyl Adamantyl
X(79Br) (MHz)
Ionic character of C-Br bond (%)
577.143 (8) a 541.7535 513.7 (8)c 503.524 (9)d
25.0 29.6 33.3 (2) 34.6
aRef. 5.5Value of Xzz(79Br) along the CBr bond direction (z) calculated from ~ , Xbb,Xab given in ref. 6. CRef. 7. dThis work.
single valence p-electron on bromine 79Br. From this simple formula and the )c(Br) values, we find that the ionic contribution to the C-Br bond in 1-bromoadamantane is slightly greater than in t-butyl bromide but significantly more so than in methyl and ethyl bromides, as shown in Table 3. We cannot comment on whether the similar but slightly larger ionic character in 1-bromoadamantane is paralleled by a flattening of the bridgehead position to which Br is attached and lengthening of the C-Br bond, as established in t-butyl bromide [7 ], because an rs-structure of 1-bromoadamantane is not yet available. ACKNOWLEDGEMENT
We thank the SERC for the award of a research studentship to A.L.W.
REFERENCES 1 2 3 4 5 6 7 8
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