Beckmann, Peter A

Wendel, Armin

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Scholarship;  Research;  and Creative Work at Bryn Mawr College

Bryn Mawr College
Scholarship, Research, and Creative Work at Bryn Mawr
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Physics Faculty Research and Scholarship Physics
1980
Proton Spin-Lattice Relaxation in Meta-Carborane
Peter A. Beckmann
Bryn Mawr College, pbeckman@brynmawr.edu
Armin Wendel
Let us know how access to this document benefits you.
Follow this and additional works at: http://repository.brynmawr.edu/physics_pubs
Part of the Physics Commons
This paper is posted at Scholarship, Research, and Creative Work at Bryn Mawr College. http://repository.brynmawr.edu/physics_pubs/15
For more information, please contact repository@brynmawr.edu.
Custom Citation
P. Beckmann and A. Wendel, J. Chem. Phys. 73, 3514 (1980).
3514 Letters to the Editor 
ground state of the exciplex (or the CT complex). This 
will render these vibrations exceptionally good accept-
ing modes for the nonradiative decay process. Deutera-
tion of the methyl group reduces the frequencies of these 
modes (especially the stretching mode), leading to a de-
crease in Franck-Condon factors for the radiationless 
process. 
The large deuterium isotope effect observed here sug-
gests that it is the changes in the methyl C -H stretching 
force constants which is largely responsible for the ob-
served isotope effect on k nr • This finding supports the 
conclusions of MO calculations8 that hyperconjugative 
interaction weakens the methyl C -H bonds. The present 
result also supports the oft-held view9 that methyl hy-
perconjugation is much more important in molecular 
cations than in their neutral precursors. 
This work was supported by the Department of Energy. 
jFor a review on deuterium effect on knr' see B. R. Henry 
and W. Siebrand, in Organic Molecular Photophysics, edited 
by J. B. Birks (Wiley-Interscience, London, 1973), Vol. 1, 
p. 153. 
2T . E. Martin and A. H. Kalantar, Chern. Phys. Lett. 4, 23 
(1969); N. G. Kilmer and J. D. Spangler, J. Chern. Phys. 
54, 604 (1971). 
3R. Li and E. C. Lim (unpublished results). 
4S. Nagakura, in Excited States, edited by E. C. Lim (Aca-
demic, New York, 1975), Vol. 2, p. 321. 
5J • Prochorow, S. Okajima, and E. C. Lim, Chern. Phys. 
Lett. 66, 590 (1979). 
6S. Okajima and E. C. Lim, Chern. Phys. Lett. 70, 283 
(1980). 
71. Deperasinska, J. Prochorow, and A. Sobolewski, Chern. 
Phys. 32, 257 (1978). 
8A. Pross, L. Radom, and N. V. Riggs, J. Am. Chern. Soc. 
102, 2253 (1980) and references therein. 
9See , for example, R. Hoffmann, L. Radom, J. A. Pople, 
P. V. R. Schleyer, W. J. Hehre, and L. Salem, J. Am. 
Chern. Soc. 94, 6221 (1972). 
Proton spin-lattice relaxation in meta-carborane 
Peter Beckmann and Armin Wendel8 ) 
Department of Physics. Graduate School of Arts and Sciences. Bryn Mawr College. Bryn Mawr. 
Pennsylvania 19010 
(Received 27 May 1980; accepted 18 June 1980) 
In a recent work l the temperature dependence of the 
proton spin-lattice relaxation time T I' was investigated 
in ortho- and para-carborane between 170 and 310 K and 
between 210 and 320 K, respectively. We have com-
pleted the present phase of the study by measuring T 1 
at 30 MHz in meta-carborane between 171 and 303 K. 
The experimental technique is discussed in Ref. 1 and 
schematic diagrams of the three isomers of carborane 
can also be found in Ref. 1. 
The experimental results for meta-carborane are 
shown in Fig. 1. There is a first order phase transition 
at about 276 K. The dashed curve below 220 K (l03 r-I 
> 4.5) in Fig. 1, shows a linear least squares fit of the 
data in this region and yields an activation energy of 25 
kJ/mol. The region on the high temperature side of the 
T 1 minimum in the low temperature phase cannot be 
fitted to a straight line because the transition occurs 
before the short correlation time limit in the low tem-
perature phase is reached. We show a dashed line cor-
responding to an activation energy of 40 kJ/mol in order 
to compare the results in meta-carborane with those of 
the other two isomers (Table II of Ref. 1). 
The details in the vicinity of the phase transition are 
shown in Fig. 2. Nonexponential relaxation is indicated 
by vertical dashed lines and is indicative of both phases 
being present. (This is discussed more fully in the 
previous work. I) Although only the short time T 1 or the 
long time T 1 was measured in most cases, for three 
runs both time constants were measured and these are 
indicated by pairs of data points connected by the verti-
cal dashed lines. The lengths of the other five vertical 
dashed lines in Fig. 2 have no significance. Taking the 
transition temperature as the midpoint of the coexistence 
range, on increasing the temperature, the transition oc-
T (K) 
300 250 200 
3 4 5 • 103T-1 (K-1) 
FIG. 1. Spin-lattice relaxation time Tj vs temperature T in 
meta-carborane. The experimental results are indicated by 
closed circles and the dashed lines are discussed in the text. 
The details in the region of the phase transition are shown in 
Fig. 2. 
J. Chern. Phys. 73(7).1 Oct. 1980 0021·9606/80/193514·02$01.00 © American I nstitute of Physics 
Downloaded 09 Feb 2012 to 165.106.1.42. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions
Letters to the Editor 3515 
1500 
1000 
1i 
(ms) 
600 
60 
30 
10 
285 
~~~I~ I Iii, 
L .... ,l", ... 
T -r ,. ... "r' 
I ' I I I 
t + 
3.5 3.6 
T (K) 
275 
~ + , , 
... ... 
.,. T 
I 
, 
r 
, 
... ,. 
, 
I 
I 
I 
I , 
, 
.-
3.7 
103 To1 (K-1) 
265 
0.6 
1.0 
1.5 
20 
so 
• 
100 
3.8 
FIG. 2. Spin-lattice relaxation time TI vs temperature Tin 
the viCinity of the phase transition in meta-carborane. Note 
that one decade is omitted on the abscissa between the T I values 
in the two phases. The circles and triangles denote decreasing 
and increasing temperature, respectively. The vertical dashed 
lines indicate nonexponential relaxation which is discussed 
further in the text. 
curs at 279.3 ± 2.8 K and on decreaSing the temperature 
it occurs at 272. 5 ± 3.9 K. The coexistence ranges are 
5. 5± 2.3 K and 10. 9± 3.4 K on increasing and decreas-
ing the temperature, respectively, and the hysteresis 
range is 6.8 ± 5.7 K. The origin of the uncertainties is 
discussed in the previous work. 1 These temperatures 
for meta -carborane can be compared with those for the 
ortho and para isomers (Table I in Ref. 1). 
The temperature dependence of T 1 in the three iso-
mers of carborane is summarized in Fig. 3. The ortho1 
and meta isomers undergo one phase transition in the 
temperature region studied whereas the para isomer1 
undergoes two transitions. The present experiments 
in meta-carborane tend to support the conjectures put 
forth in the previous work1 in that the high temperature 
phase transition in meta-carborane occurs at a tempera-
ture between that found for the other two isomers and at 
low temperatures, meta-carborane behaves like ortho-
carborane and not like para-carborane, particularly with 
respect to the lack of a second phase transition. The 
origin of the difference between the T 1 vs T curves for 
ortho- and para-carborane is discussed in detail in the 
Tl 
(5) 
3 
300 
T (K) 
2SO 
/ 
/ 
/ 
/ 
I 
I 
I 
I 
I 
I 
I 
I 
I 
~/,/ 
/ 
/ 
/ 
/ 
I 
I " 
, 
, 
I 
I 
I 
'1 
f 
200 
'/ 
'/ 
'1 
4 5 
103T-1 (Ko1) 
I 
/ 
/ 
~ 
~ 
'1 
6 
10 
FIG. 3. A summaty of the temperature dependence of the 
spin-lattice relaxation times T I , in the carboranes. The solid 
line corresponds to ortho-carborane, the long d8shes cor-
respond to meta-carborane and the short dashes correspond 
to para-carborane. The results for the ortho and para isomers 
are taken from Fig. 2 of Ref. 1 and the results for the meta 
isomer are taken from Fig. 1 of this work. Note that ortho-
and meta-carborane experience one phase transition whereas 
para-carborane experiences two transitions. 
previous work1 and is based on the fact that the ortho 
(and meta) isomers have a relatively large molecular 
electric dipole moment whereas para-carborane has 
none. 
There are important problems which still require in-
vestigation in the carborane system and these are dis-
cussed in the previous work. 1 
We are grateful to Professor Amos Leffler of Villanova 
University for supplying the meta-carborane. 
a)Permanent address: Kantonsschule Kreuzlingen, CH-8280 
Kreuzlingen, Switzerland. 
Ip. A. Beckmann and A. J. Leffler, J. Chern. Phys. 72, 
4600 (1980). 
Spin-orbit coupling and A -doubling in LiO 
David L. Copper and W. Graham Richards 
Physical Chemistry Laboratory. South Parks Road. Oxford. U. K. 
(Received 15 May 1980; accepted 16June 1980) 
Theoretical calculations of molecular spectroscopic 
properties have reached a stage where in some instances 
they challenge experimental determinations. This is 
true even in the accurate radio frequency region since 
parameters are derived by fitting observed transitions 
using formulas which necessitate assumptions about 
J. Chern. Phys. 73(7). 1 Oct. 1980 0021-9606/80/193515-03$0 1.00 © American I nstitute of Physics 
Downloaded 09 Feb 2012 to 165.106.1.42. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions


Proton Spin-Lattice Relaxation in Meta-Carborane

Scholarship, Research, and Creative Work at Bryn Mawr College | Bryn Mawr College Research

Bryn Mawr CollegeScholarship, Research, and Creative Work at Bryn MawrCollegePhysics Faculty Research and Scholarship Physics1980Proton Spin-Lattice Relaxation in Meta-CarboranePeter A. BeckmannBryn Mawr College, pbeckman@brynmawr.eduArmin WendelLet us know how access to this document benefits you.Follow this and additional works at: http://repository.brynmawr.edu/physics_pubsPart of the Physics CommonsThis paper is posted at Scholarship, Research, and Creative Work at Bryn Mawr College. http://repository.brynmawr.edu/physics_pubs/15For more information, please contact repository@brynmawr.edu.Custom CitationP. Beckmann and A. Wendel, J. Chem. Phys. 73, 3514 (1980).3514 Letters to the Editor ground state of the exciplex (or the CT complex). This will render these vibrations exceptionally good accept-ing modes for the nonradiative decay process. Deutera-tion of the methyl group reduces the frequencies of these modes (especially the stretching mode), leading to a de-crease in Franck-Condon factors for the radiationless process. The large deuterium isotope effect observed here sug-gests that it is the changes in the methyl C -H stretching force constants which is largely responsible for the ob-served isotope effect on k nr • This finding supports the conclusions of MO calculations8 that hyperconjugative interaction weakens the methyl C -H bonds. The present result also supports the oft-held view9 that methyl hy-perconjugation is much more important in molecular cations than in their neutral precursors. This work was supported by the Department of Energy. jFor a review on deuterium effect on knr' see B. R. Henry and W. Siebrand, in Organic Molecular Photophysics, edited by J. B. Birks (Wiley-Interscience, London, 1973), Vol. 1, p. 153. 2T . E. Martin and A. H. Kalantar, Chern. Phys. Lett. 4, 23 (1969); N. G. Kilmer and J. D. Spangler, J. Chern. Phys. 54, 604 (1971). 3R. Li and E. C. Lim (unpublished results). 4S. Nagakura, in Excited States, edited by E. C. Lim (Aca-demic, New York, 1975), Vol. 2, p. 321. 5J • Prochorow, S. Okajima, and E. C. Lim, Chern. Phys. Lett. 66, 590 (1979). 6S. Okajima and E. C. Lim, Chern. Phys. Lett. 70, 283 (1980). 71. Deperasinska, J. Prochorow, and A. Sobolewski, Chern. Phys. 32, 257 (1978). 8A. Pross, L. Radom, and N. V. Riggs, J. Am. Chern. Soc. 102, 2253 (1980) and references therein. 9See , for example, R. Hoffmann, L. Radom, J. A. Pople, P. V. R. Schleyer, W. J. Hehre, and L. Salem, J. Am. Chern. Soc. 94, 6221 (1972). Proton spin-lattice relaxation in meta-carborane Peter Beckmann and Armin Wendel8 ) Department of Physics. Graduate School of Arts and Sciences. Bryn Mawr College. Bryn Mawr. Pennsylvania 19010 (Received 27 May 1980; accepted 18 June 1980) In a recent work l the temperature dependence of the proton spin-lattice relaxation time T I' was investigated in ortho- and para-carborane between 170 and 310 K and between 210 and 320 K, respectively. We have com-pleted the present phase of the study by measuring T 1 at 30 MHz in meta-carborane between 171 and 303 K. The experimental technique is discussed in Ref. 1 and schematic diagrams of the three isomers of carborane can also be found in Ref. 1. The experimental results for meta-carborane are shown in Fig. 1. There is a first order phase transition at about 276 K. The dashed curve below 220 K (l03 r-I > 4.5) in Fig. 1, shows a linear least squares fit of the data in this region and yields an activation energy of 25 kJ/mol. The region on the high temperature side of the T 1 minimum in the low temperature phase cannot be fitted to a straight line because the transition occurs before the short correlation time limit in the low tem-perature phase is reached. We show a dashed line cor-responding to an activation energy of 40 kJ/mol in order to compare the results in meta-carborane with those of the other two isomers (Table II of Ref. 1). The details in the vicinity of the phase transition are shown in Fig. 2. Nonexponential relaxation is indicated by vertical dashed lines and is indicative of both phases being present. (This is discussed more fully in the previous work. I) Although only the short time T 1 or the long time T 1 was measured in most cases, for three runs both time constants were measured and these are indicated by pairs of data points connected by the verti-cal dashed lines. The lengths of the other five vertical dashed lines in Fig. 2 have no significance. Taking the transition temperature as the midpoint of the coexistence range, on increasing the temperature, the transition oc-T (K) 300 250 200 3 4 5 • 103T-1 (K-1) FIG. 1. Spin-lattice relaxation time Tj vs temperature T in meta-carborane. The experimental results are indicated by closed circles and the dashed lines are discussed in the text. The details in the region of the phase transition are shown in Fig. 2. J. Chern. Phys. 73(7).1 Oct. 1980 0021·9606/80/193514·02$01.00 © American I nstitute of Physics Downloaded 09 Feb 2012 to 165.106.1.42. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsLetters to the Editor 3515 1500 1000 1i (ms) 600 60 30 10 285 ~~~I~ I Iii, L .... ,l", ... T -r ,. ... "r' I ' I I I t + 3.5 3.6 T (K) 275 ~ + , , ... ... .,. T I , r , ... ,. , I I I I , , .-3.7 103 To1 (K-1) 265 0.6 1.0 1.5 20 so • 100 3.8 FIG. 2. Spin-lattice relaxation time TI vs temperature Tin the viCinity of the phase transition in meta-carborane. Note that one decade is omitted on the abscissa between the T I values in the two phases. The circles and triangles denote decreasing and increasing temperature, respectively. The vertical dashed lines indicate nonexponential relaxation which is discussed further in the text. curs at 279.3 ± 2.8 K and on decreaSing the temperature it occurs at 272. 5 ± 3.9 K. The coexistence ranges are 5. 5± 2.3 K and 10. 9± 3.4 K on increasing and decreas-ing the temperature, respectively, and the hysteresis range is 6.8 ± 5.7 K. The origin of the uncertainties is discussed in the previous work. 1 These temperatures for meta -carborane can be compared with those for the ortho and para isomers (Table I in Ref. 1). The temperature dependence of T 1 in the three iso-mers of carborane is summarized in Fig. 3. The ortho1 and meta isomers undergo one phase transition in the temperature region studied whereas the para isomer1 undergoes two transitions. The present experiments in meta-carborane tend to support the conjectures put forth in the previous work1 in that the high temperature phase transition in meta-carborane occurs at a tempera-ture between that found for the other two isomers and at low temperatures, meta-carborane behaves like ortho-carborane and not like para-carborane, particularly with respect to the lack of a second phase transition. The origin of the difference between the T 1 vs T curves for ortho- and para-carborane is discussed in detail in the Tl (5) 3 300 T (K) 2SO / / / / I I I I I I I I I ~/,/ / / / / I I " , , I I I '1 f 200 '/ '/ '1 4 5 103T-1 (Ko1) I / / ~ ~ '1 6 10 FIG. 3. A summaty of the temperature dependence of the spin-lattice relaxation times T I , in the carboranes. The solid line corresponds to ortho-carborane, the long d8shes cor-respond to meta-carborane and the short dashes correspond to para-carborane. The results for the ortho and para isomers are taken from Fig. 2 of Ref. 1 and the results for the meta isomer are taken from Fig. 1 of this work. Note that ortho-and meta-carborane experience one phase transition whereas para-carborane experiences two transitions. previous work1 and is based on the fact that the ortho (and meta) isomers have a relatively large molecular electric dipole moment whereas para-carborane has none. There are important problems which still require in-vestigation in the carborane system and these are dis-cussed in the previous work. 1 We are grateful to Professor Amos Leffler of Villanova University for supplying the meta-carborane. a)Permanent address: Kantonsschule Kreuzlingen, CH-8280 Kreuzlingen, Switzerland. Ip. A. Beckmann and A. J. Leffler, J. Chern. Phys. 72, 4600 (1980). Spin-orbit coupling and A -doubling in LiO David L. Copper and W. Graham Richards Physical Chemistry Laboratory. South Parks Road. Oxford. U. K. (Received 15 May 1980; accepted 16June 1980) Theoretical calculations of molecular spectroscopic properties have reached a stage where in some instances they challenge experimental determinations. This is true even in the accurate radio frequency region since parameters are derived by fitting observed transitions using formulas which necessitate assumptions about J. Chern. Phys. 73(7). 1 Oct. 1980 0021-9606/80/193515-03$0 1.00 © American I nstitute of Physics Downloaded 09 Feb 2012 to 165.106.1.42. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

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Proton Spin-Lattice Relaxation in Meta-Carborane

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Scholarship; Research; and Creative Work at Bryn Mawr College

Scholarship, Research, and Creative Work at Bryn Mawr College | Bryn Mawr College Research