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The molecular zeeman effect in propene and comparison of the magnetic susceptibility anisotropies with cyclopropene and other small ring compounds
Volume
CHEMICALPHYSICSLETTERS
4. number 3
THE
MOLECULAR OF WITH
THE
ZEEMAN MAGNETIC
CYCLOPROPENE
EFFECT
IN PROPENE
SUSCEPTIBILITY AND
I5 October 1969
OTHER
AND
COMPARISON
ANISOTROPIES SMALL
RING
R. C. BENSON and W. II. FLYGARE Noyes Chemical Laboratoy,. University of Illinois, U&ma.
COMPOUNDS
Illinois.
UsA
Received 31 July 1969 The molecular Zeemand effect has been observed in propene in magnetic fields of 21 kG. The molecular g-values are gas = -0.0789 l 0.0006. gbb = -0.0424 f 0.0004, and&, = +0.0197 f 0.0005. The magnetic susceptibility aniSOt.rOpieS are 2X== -Xbb - Xcc = -0.14 f 0.30 and 2Xbb - & - X,, = +13.4 + 0.5 in units of 10m6 erg/& mole where the c axis is out of plane ixd the b axis nearly bisects the CCC angle. The molecular auadruoole moments are also obtained. The magnetic susceptibility anisotropies are compared with other molecules. We have observed the molecular Zeeman effect in propene at high magnetic fields which gives the three molecularg-values (gag, gbb, and gee) and the two independent values for the magnetic susceptibility anisotropy ((2xQa - Xbb These Zeeman pa- X,,) a-d (2xbb- &a - x,,)). rameters lead to a direct determination of the molecular electric quadrupole moments (Qaa, Qbb, and Qcc). The results in propene show that the electric dipole and quadrupole moments are very similar to the values in cyclopropene. However, the magnetic susceptibility anisotropies in propene are much smaller than in cyclopropene. There appears to be a considerable non-local (ring current) effect contributing to the magnetic susceptibility anisotropy in cyclopropene relative to propene. The results obtained here are also compared to other small rings and other planar molecules. Recently we have developed the theoretical [l] and experimental [2,3] basis for measuring magnetic susceptibility anisotropies by using high magnetic fields and high-resolution microwave spectroscopy. The molecular Zeeman effect in several small ring compounds has been reported which include the molecular g-values, magnetic susceptibility anisotropies, and molecular quadrupole moments. These molecules include fluorbenzene [4], ethylene oxide [6], ethylene sulfide [6], thiophene [‘I], furan [7], and cyclopropene [8]. There has been a major interest and considerable controversy in interpreting the magnetic susceptibility anisotropies in ring compounds in terms of a combination of ring CUT-
rents [S-15] and local effects [16,17]. Indirect measurements of ring current and local magnetic anisotropies have been given on the basis of proton chemical shifts *. However, these methods do not lead to unambiguous results as found in arguments on the magnitude of the ring currents in the benzene, thiophene, pyrolle, and furan series [12,15,19-211. Our previous work on the above ring compounds provides the first direct experimental measurements of the magnetic susceptibility anisotropies **. The microwave spectrum and molecular structure of propene have been previously determined [23,24]. The AMJ = O,*l Zeeman transitions of the 000 - 101, iii212, Loi202, and 110- 211 rotational transitions were studied at a magnetic field of about 21000G. The data for both the A- and E-levels (due to internal rotation) were analyzed by least squares in the usual manner [2-81 to give the five molecular Zeeman Parameters (g-, g& gee, 2Xa(~-Xbb -Xcc, -Xm + 2Xbb - X,o) listed in table 1. The molecular quadrupole moments, Qxxl are obtained directly from the above numbers. The known molecular structure [24] is then combined * A summary of this method is given by Pople et dl. fls). For more current work see ref. [I31 and 1141. ** Orientated crystal measurements have yielded direct measurements in the solid state of the magnetic susceptibility anisotropies in benzene and other larger compounds (see a review in [22]). AIL of the microwave work discussed here is done witt the sample in the gas phase at low pressures-
141
CHEMICAL
vobnne 4, number 3
PHYSICS
Table 1 Zeeman parameters, molecular quadrupoie moments, and second moments of the electronic charge distribution in propene. The a and b axes are in the molecular plane. The magnetic susceptibilities are listed in units of 10-6 erg/G2 mole, the molecular quadrupole moments are in units of 10-26 esu cm2, and the second moments are in units of 10-16 cm2. 6
/b/
C
gaa = -0.0789 +z0.0006 Rhh
=
-0.0424 f 0.0004
- +0.0107 * 0.0005 ;F -- ,&,- x, = -0.74 f 0.3 -Yv
+=&+2xbb - & 0.6 -L 0.3 _--
6thh
=
2.9
f
= t13.4 f 0.5
0.5
a
‘$(Xss
+ Xbb +&c) = -30.7 f 0.8
XdU = -73.3 f 1.3 xd,, = -184.8 * 1.6 tic,,
=
-206.6 + 1.7
Xm
=
-30.9
* 0.9
xbb = -26.3 * 1.0 &
= 42.4 f 0.4
158.5 * 0.6 xgb = xp = 171.7 * 0.6 CC
the Zeeman parameters to give the diagonal elements in the paramagnetic susceptibility tensor, xi&, and the anisotropies in the second moment of the electronic charge distribution, (x2) - (y2> *. All of these results are listed in the left hand column of table 1. The bulk susceptibility in the gas phase has been measured [25] and it is used to extract the remaining values listed in the right hand column of table 1 [25]. The magnetic susceptibility anisotropies in propene are considerabiy smaller than in cyclopropene. The values 01 xzz - +(xXx + xYY) are listed in table 2 for a number of small molecules. The z axis is perpendicular to the molecular plane and the in-plane value of (xX.+x ,,) is invariant to rotation about.the z axis.-T x e lowering of the magnitude of xzz - Bo(,, + xyu) from -17.0 * 0.5 X 10m6 erg,@ mole in cyclopropene to -6.3 f 0.4 x 10-6 erg/& mole in propene indicates a dramatic lowering of the magnetic susceptibility anisotropy when the three-membered ring is opened. This change in anisotropy could arise from either a change in local carbon hybridization or a change in the non-local properties. The vinyl carbon atoms in propene apparently have near s$ hybridization as determined frcm the JHl3C spin-spin coupling constants
LETTER3
15 October 1969
Table 2 - Xy,,) in units of lOa erg/G2 mole for some small rings and other planar molecules. The z axis is perpendioulsr to the heavy atom plane
Values of Xzz - 4(X,
Molecule
x22-&&r
benzene fluorobenzene thiophene pyrolle furan cyclopropene ethylene sulfide
ethylene imine ethylene oxide 1.3-cyclohexadiene ketene vinylidene fluoride cis-1.2~ciifIuoroethylene propene a.
6. c. d. e.
Ref.
-Xyy)
-59.7 -58.3 + 0.8
-50.1
l
1.0
-42.4 -33.7 -17.0 -15.4
* * f *
0.5 0.5 0.5 0.4
-10.9 -9-4 -7.4 2.6 -2.7 -2.2 -6.3
l 0.7 f o-4 * 1.8 * 0.6 t 0.7 IO.4 rt 0.4
: h’ 7 8
: 5 : e t&S
tK3d.Z
J_Hoarau. N. Lumbroso and A. Pecauit, Compt. Rend. Aced. Sci. (Paris) 242 (l9.56) 1702. D. H. Sutter end W. H. Flygare. unpublished results. J. M. Pochan and W. H. Flygare. unpublished results. W. Htittuer, P. D. Foster and W. H.Flygare, J. Chem. Phys. 50 (1969) 1710. R. P. Blickensderfer, J. H.S. Wang and W.H.Flygare, J. Chem. Phys., to be published.
with
* See refs. [l-8]
calculations.
142
for the equations necessary for these
[26,27]. The hybridization of the carbon atoms in the cyclopropene molecule is apparently more nearly sp [28-301. However, a detailed study of magnetic susceptibilities as a function of carbon hybridization [31] indicates that this local change will not lead to the large differences in molecular anisotropies observed here between propene and cyclopropene. Thus, we attribute the large differences in the anisotropies to non-local effects which are surprisingly large for the unconjugated cyclopropene ring relative to propene. Details of the work on propene and further work on dimethyl ether and dimethyl sulfide and comparison with the ring closed ethylene oxide and etylene sulfide (see table 2) molecules will be given at a later time. The support of the National Science Foundation is gratefully acknowledged.
REFERENCES [l] W. Hiittner and W.H.Flygare,
J. Chem. Phys. 47 (1967) 4137. -[2] W. Hiittner, M.K. Lo and W.H.Flygare, J. Chem. Phys. 48 (1968) 1206. (31 W. H. Flygsre, W.HUttner. R. L-Shoemaker and P,D.Foster, J..Chem. Phys. 50 (1969) 1714.
Volume 4, number 3
CHEMICAL PHYSICS LETTERS
[C] W.HUttner and W.H.Flygare, J. Chem. Phys. 50 (1969) 2863. [5] D.H.Sutter, W.HUttner and W.H.Flygare, J. Chem. Phys. 50 (1969) 2869. [S] D.H.Sutter and W.H.Flygare. Mol. Phys. 16 (1969) 153. [q D.H.Sutter and W.H. Flygare, J. Am. Chem. Sot. to be published. 181 R.C.Benson and W. H. Flygare, J. Chem. Phys.. to be oublished. [9] L.P&ling, J. Chem. Phys. 4 (1933) 673. 1101 F.London. J. Phvs. Radium 8 (193n 397. illi J.S. Waugh and R’. W. Fessenden. J.‘Am. Chem. Sot. 79 (1957) 846. [12] D.W.Davies, Trans. Faraday Sot. 57 (1961) 2081. 1131 B.P.Dailey, J. Chem. Phys. 41 (1964) 2304. [14] A.F. Furguson and J.A.Pople, J. Cbem. Phys. 42 (1966) 1.560 and earlier references cited therein. [IS] P.J.Black. R.D.Brown and M.L.Heffernan, Australian J. Chem. 20 (1967) 1305. 1161 J. Hoar-au, Ann. Chim. (Paris) l(l956) 560. [17] J.I.Musher, J. Chem. Phys. 43 (1965) 4081; J.M.Gaidis and R. West, J. Chem. Phys. 46 (1967) 1218; J.I.Musher, J. Chem. Phys. 46 (1967) 1219. 1181 J.A.Pople, W.G.Schneider and J.H.Bernstein High resolution nuclear magnetic resonance
I.5 October 1969
(McGraw-HiI& New York, 1959). [19] J.A.FLvidge and L.M.Jackmsn. 6. Chem. Sac. (1961) 859. [ZOJ R.J.Abraham. R.C.Sheppnrd. W-A-Thomas and S. Turner, Chem. Commun. (1965) 43. [Zl] J.A. Elvidge. Chem. Cammun. (1965) 160. 1221 A. A. Bothner-By and J. A. Pople, Ann. Rev. Phys. Chem. 16 (1965) 43. 1231 D.R.Lide and D-E-Mann, J. Chem. Phys. 27 (1957) 868. 1241 D. R. Lide and D. Christensen, J_ ChembPhys. 35 (1961) 1374. [25j C.Barter. R.G_Meisenheimer and D.P.Steven&n, J. Phys. Chem. 64 (1960) 1312. [26] C. Juan and H. S.Gutowsky. J. Chem. Phys. 37 (1962) 2198 and references cited theretn. [27’l A.A.kothner-By and C.Naar-Colin, J. Am. Chem. Sot. 83 (1961) 231. [28] G. L. Gloss and R.B.Larrabee. Tetrahedron Letters (1967) 287. WI M. K.Kemp and W. H. Flygare. J. Am. Chem. Sot. 91 (1969) 3163. 1301 K.B. Wiberg and B. J.Nist, J. Am. Chem. Sot. 63 (1961) 1226. [311 A. Pacault, J. Hoarau and A. Marchand, Adwn. Chem. Phys. 3 (1961) 17l.
143
CHEMICALPHYSICSLETTERS
4. number 3
THE
MOLECULAR OF WITH
THE
ZEEMAN MAGNETIC
CYCLOPROPENE
EFFECT
IN PROPENE
SUSCEPTIBILITY AND
I5 October 1969
OTHER
AND
COMPARISON
ANISOTROPIES SMALL
RING
R. C. BENSON and W. II. FLYGARE Noyes Chemical Laboratoy,. University of Illinois, U&ma.
COMPOUNDS
Illinois.
UsA
Received 31 July 1969 The molecular Zeemand effect has been observed in propene in magnetic fields of 21 kG. The molecular g-values are gas = -0.0789 l 0.0006. gbb = -0.0424 f 0.0004, and&, = +0.0197 f 0.0005. The magnetic susceptibility aniSOt.rOpieS are 2X== -Xbb - Xcc = -0.14 f 0.30 and 2Xbb - & - X,, = +13.4 + 0.5 in units of 10m6 erg/& mole where the c axis is out of plane ixd the b axis nearly bisects the CCC angle. The molecular auadruoole moments are also obtained. The magnetic susceptibility anisotropies are compared with other molecules. We have observed the molecular Zeeman effect in propene at high magnetic fields which gives the three molecularg-values (gag, gbb, and gee) and the two independent values for the magnetic susceptibility anisotropy ((2xQa - Xbb These Zeeman pa- X,,) a-d (2xbb- &a - x,,)). rameters lead to a direct determination of the molecular electric quadrupole moments (Qaa, Qbb, and Qcc). The results in propene show that the electric dipole and quadrupole moments are very similar to the values in cyclopropene. However, the magnetic susceptibility anisotropies in propene are much smaller than in cyclopropene. There appears to be a considerable non-local (ring current) effect contributing to the magnetic susceptibility anisotropy in cyclopropene relative to propene. The results obtained here are also compared to other small rings and other planar molecules. Recently we have developed the theoretical [l] and experimental [2,3] basis for measuring magnetic susceptibility anisotropies by using high magnetic fields and high-resolution microwave spectroscopy. The molecular Zeeman effect in several small ring compounds has been reported which include the molecular g-values, magnetic susceptibility anisotropies, and molecular quadrupole moments. These molecules include fluorbenzene [4], ethylene oxide [6], ethylene sulfide [6], thiophene [‘I], furan [7], and cyclopropene [8]. There has been a major interest and considerable controversy in interpreting the magnetic susceptibility anisotropies in ring compounds in terms of a combination of ring CUT-
rents [S-15] and local effects [16,17]. Indirect measurements of ring current and local magnetic anisotropies have been given on the basis of proton chemical shifts *. However, these methods do not lead to unambiguous results as found in arguments on the magnitude of the ring currents in the benzene, thiophene, pyrolle, and furan series [12,15,19-211. Our previous work on the above ring compounds provides the first direct experimental measurements of the magnetic susceptibility anisotropies **. The microwave spectrum and molecular structure of propene have been previously determined [23,24]. The AMJ = O,*l Zeeman transitions of the 000 - 101, iii212, Loi202, and 110- 211 rotational transitions were studied at a magnetic field of about 21000G. The data for both the A- and E-levels (due to internal rotation) were analyzed by least squares in the usual manner [2-81 to give the five molecular Zeeman Parameters (g-, g& gee, 2Xa(~-Xbb -Xcc, -Xm + 2Xbb - X,o) listed in table 1. The molecular quadrupole moments, Qxxl are obtained directly from the above numbers. The known molecular structure [24] is then combined * A summary of this method is given by Pople et dl. fls). For more current work see ref. [I31 and 1141. ** Orientated crystal measurements have yielded direct measurements in the solid state of the magnetic susceptibility anisotropies in benzene and other larger compounds (see a review in [22]). AIL of the microwave work discussed here is done witt the sample in the gas phase at low pressures-
141
CHEMICAL
vobnne 4, number 3
PHYSICS
Table 1 Zeeman parameters, molecular quadrupoie moments, and second moments of the electronic charge distribution in propene. The a and b axes are in the molecular plane. The magnetic susceptibilities are listed in units of 10-6 erg/G2 mole, the molecular quadrupole moments are in units of 10-26 esu cm2, and the second moments are in units of 10-16 cm2. 6
/b/
C
gaa = -0.0789 +z0.0006 Rhh
=
-0.0424 f 0.0004
- +0.0107 * 0.0005 ;F -- ,&,- x, = -0.74 f 0.3 -Yv
+=&+2xbb - & 0.6 -L 0.3 _--
6thh
=
2.9
f
= t13.4 f 0.5
0.5
a
‘$(Xss
+ Xbb +&c) = -30.7 f 0.8
XdU = -73.3 f 1.3 xd,, = -184.8 * 1.6 tic,,
=
-206.6 + 1.7
Xm
=
-30.9
* 0.9
xbb = -26.3 * 1.0 &
= 42.4 f 0.4
158.5 * 0.6 xgb = xp = 171.7 * 0.6 CC
the Zeeman parameters to give the diagonal elements in the paramagnetic susceptibility tensor, xi&, and the anisotropies in the second moment of the electronic charge distribution, (x2) - (y2> *. All of these results are listed in the left hand column of table 1. The bulk susceptibility in the gas phase has been measured [25] and it is used to extract the remaining values listed in the right hand column of table 1 [25]. The magnetic susceptibility anisotropies in propene are considerabiy smaller than in cyclopropene. The values 01 xzz - +(xXx + xYY) are listed in table 2 for a number of small molecules. The z axis is perpendicular to the molecular plane and the in-plane value of (xX.+x ,,) is invariant to rotation about.the z axis.-T x e lowering of the magnitude of xzz - Bo(,, + xyu) from -17.0 * 0.5 X 10m6 erg,@ mole in cyclopropene to -6.3 f 0.4 x 10-6 erg/& mole in propene indicates a dramatic lowering of the magnetic susceptibility anisotropy when the three-membered ring is opened. This change in anisotropy could arise from either a change in local carbon hybridization or a change in the non-local properties. The vinyl carbon atoms in propene apparently have near s$ hybridization as determined frcm the JHl3C spin-spin coupling constants
LETTER3
15 October 1969
Table 2 - Xy,,) in units of lOa erg/G2 mole for some small rings and other planar molecules. The z axis is perpendioulsr to the heavy atom plane
Values of Xzz - 4(X,
Molecule
x22-&&r
benzene fluorobenzene thiophene pyrolle furan cyclopropene ethylene sulfide
ethylene imine ethylene oxide 1.3-cyclohexadiene ketene vinylidene fluoride cis-1.2~ciifIuoroethylene propene a.
6. c. d. e.
Ref.
-Xyy)
-59.7 -58.3 + 0.8
-50.1
l
1.0
-42.4 -33.7 -17.0 -15.4
* * f *
0.5 0.5 0.5 0.4
-10.9 -9-4 -7.4 2.6 -2.7 -2.2 -6.3
l 0.7 f o-4 * 1.8 * 0.6 t 0.7 IO.4 rt 0.4
: h’ 7 8
: 5 : e t&S
tK3d.Z
J_Hoarau. N. Lumbroso and A. Pecauit, Compt. Rend. Aced. Sci. (Paris) 242 (l9.56) 1702. D. H. Sutter end W. H. Flygare. unpublished results. J. M. Pochan and W. H. Flygare. unpublished results. W. Htittuer, P. D. Foster and W. H.Flygare, J. Chem. Phys. 50 (1969) 1710. R. P. Blickensderfer, J. H.S. Wang and W.H.Flygare, J. Chem. Phys., to be published.
with
* See refs. [l-8]
calculations.
142
for the equations necessary for these
[26,27]. The hybridization of the carbon atoms in the cyclopropene molecule is apparently more nearly sp [28-301. However, a detailed study of magnetic susceptibilities as a function of carbon hybridization [31] indicates that this local change will not lead to the large differences in molecular anisotropies observed here between propene and cyclopropene. Thus, we attribute the large differences in the anisotropies to non-local effects which are surprisingly large for the unconjugated cyclopropene ring relative to propene. Details of the work on propene and further work on dimethyl ether and dimethyl sulfide and comparison with the ring closed ethylene oxide and etylene sulfide (see table 2) molecules will be given at a later time. The support of the National Science Foundation is gratefully acknowledged.
REFERENCES [l] W. Hiittner and W.H.Flygare,
J. Chem. Phys. 47 (1967) 4137. -[2] W. Hiittner, M.K. Lo and W.H.Flygare, J. Chem. Phys. 48 (1968) 1206. (31 W. H. Flygsre, W.HUttner. R. L-Shoemaker and P,D.Foster, J..Chem. Phys. 50 (1969) 1714.
Volume 4, number 3
CHEMICAL PHYSICS LETTERS
[C] W.HUttner and W.H.Flygare, J. Chem. Phys. 50 (1969) 2863. [5] D.H.Sutter, W.HUttner and W.H.Flygare, J. Chem. Phys. 50 (1969) 2869. [S] D.H.Sutter and W.H.Flygare. Mol. Phys. 16 (1969) 153. [q D.H.Sutter and W.H. Flygare, J. Am. Chem. Sot. to be published. 181 R.C.Benson and W. H. Flygare, J. Chem. Phys.. to be oublished. [9] L.P&ling, J. Chem. Phys. 4 (1933) 673. 1101 F.London. J. Phvs. Radium 8 (193n 397. illi J.S. Waugh and R’. W. Fessenden. J.‘Am. Chem. Sot. 79 (1957) 846. [12] D.W.Davies, Trans. Faraday Sot. 57 (1961) 2081. 1131 B.P.Dailey, J. Chem. Phys. 41 (1964) 2304. [14] A.F. Furguson and J.A.Pople, J. Cbem. Phys. 42 (1966) 1.560 and earlier references cited therein. [IS] P.J.Black. R.D.Brown and M.L.Heffernan, Australian J. Chem. 20 (1967) 1305. 1161 J. Hoar-au, Ann. Chim. (Paris) l(l956) 560. [17] J.I.Musher, J. Chem. Phys. 43 (1965) 4081; J.M.Gaidis and R. West, J. Chem. Phys. 46 (1967) 1218; J.I.Musher, J. Chem. Phys. 46 (1967) 1219. 1181 J.A.Pople, W.G.Schneider and J.H.Bernstein High resolution nuclear magnetic resonance
I.5 October 1969
(McGraw-HiI& New York, 1959). [19] J.A.FLvidge and L.M.Jackmsn. 6. Chem. Sac. (1961) 859. [ZOJ R.J.Abraham. R.C.Sheppnrd. W-A-Thomas and S. Turner, Chem. Commun. (1965) 43. [Zl] J.A. Elvidge. Chem. Cammun. (1965) 160. 1221 A. A. Bothner-By and J. A. Pople, Ann. Rev. Phys. Chem. 16 (1965) 43. 1231 D.R.Lide and D-E-Mann, J. Chem. Phys. 27 (1957) 868. 1241 D. R. Lide and D. Christensen, J_ ChembPhys. 35 (1961) 1374. [25j C.Barter. R.G_Meisenheimer and D.P.Steven&n, J. Phys. Chem. 64 (1960) 1312. [26] C. Juan and H. S.Gutowsky. J. Chem. Phys. 37 (1962) 2198 and references cited theretn. [27’l A.A.kothner-By and C.Naar-Colin, J. Am. Chem. Sot. 83 (1961) 231. [28] G. L. Gloss and R.B.Larrabee. Tetrahedron Letters (1967) 287. WI M. K.Kemp and W. H. Flygare. J. Am. Chem. Sot. 91 (1969) 3163. 1301 K.B. Wiberg and B. J.Nist, J. Am. Chem. Sot. 63 (1961) 1226. [311 A. Pacault, J. Hoarau and A. Marchand, Adwn. Chem. Phys. 3 (1961) 17l.
143