Introduzione

1
The structure and from the NMR
G. Celebrea,
2
conformation of flexible spectra of
samples disso crystalline solvents.
G. De Lucaa, M. Longeria, G. Pileioa,
aDipartimento di Chimica, Università della
Calabria, Italy. bDepartment of Chemistry,
University of Southampton, UK.
3
molecules obtained lved in liquid
J.W. Emsleyb.
4
Introduction
A new method is described for relating dipolar
couplings Dij to the conformation of molecules
dissolved in Liquid Crystalline solvents1. We
present an example of applying this NMR method to
the flexible molecule diphenylmethane (DPM) which
has two coupled rotors.
Theory2
5
Figure 1 Structure of diphenylmethane (DPM)

Conformation Point Group t1 t2 0, 90
C2V t1 0 , t2 90 CS t1 -
t2 CS t1 t2
C2 t1 ? t2 C1
t t1 t2 t- t1 t2
6
Model for PLC(tk) and Saa(tk)

• Several possible models have been
• proposed. The AP2 method defines

where
This allow the definition of two probability
functions
with
and
with
7
The order parameters Saa(tk) are obtained as
For flexible molecule the
is modelled as
and usuallyUint(tk) is expanded in a Fourier
series such as
with n taking values to give the correct
periodicity of Uint(tk) and the series being kept
as short as will represent the correct overall
shape. For molecules with two or more internal
coupled rotors like DPM this form is not able to
correctly describe the energy surface.
8
We introduce a new method where the distribution
PLC(t1,t2) is given directly as a product of
gaussian functions
with
is position of maximum
h is width along t h- width along t-
9
In the figures are reported respectively the map
of potential curves V(t1 ,t2 ) obtained from
CVFF 3 (figure 2) and the map of level curves
of the function PLC(t1 ,t2 ) (figure 3).
Figure 3
Figure 2
10
1H spectrum of 12 interacting protons is very
complex
Figure 4
NOT YET ANALYSED
A general way to extracting dipolar couplings
between protons is to simplify the spectrum by
preparing partially-deuteriated compounds and
analysing 1H 2H spectra 4. In the figures
5 and 6 are reported the structure and the
spectra of isotopomers required for the molecule
DPM.
11
Analysed
Figure 5
Analysed
Figure 6
12
 
 
 
 
In the table are reported the experimental values
of the 13 Dij obtained from the analysis of the
spectrum in figure 6 (column A) and the
calculated values of the Dij by the AP method
using the MNDO potential surface (column B ),
the B3LYP/6-31G potential surface5 (column C )
and the PLC (t ,t- ) (column D).
 
 
13
In the table are reported the interaction
coefficients , and parameters obtained by
fitting observed dipolar couplings with
PLC(t,t-). In the figure 7 is reported the
conformation distribution the maximum of
PLC(t,t-) is for t1 t2 56.5 with C2 symmetry
for the conformer at the minimum.
Figure 7
14
Effect of vibrational motion6
Since DPM has just one conformer with a large
spread in t and t- , it is interesting to see if
an approximate model fits the data which assumes
harmonic vibrations about the position t1 t2
56.5. Vibrational corrections were calculated
theoretically using wavefunctions obtained by the
B3LYP method, together with experimental
frequencies7 Using all the vibrational modes
and varying the parametrs , ,,
, , , and produces unacceptable
value of the RMS. This large value of RMS is
probably because the effect of the oscillation
involving t1 and t2 is counted twice. By removing
the torsional modes and repeating the
optimisation we obtained a better fit, with
almost the same maximum (t1 t2 55.1) but
slight different shape of the probability
distribution.
15
In the table are reported the value of
parameters without vibrational corrections
(column A), with all the frequencies (column B)
and without torsional frequencies (column C). In
the figure 8 is reported the conformation
distribution for case C.
Figure 8
16
References
1 J. W. Emsley in Encyclopedia of NMR ,
Eds D. M. Grant and R. K. Harris, Wiley, New
York, 1996. 2 J. W. Emsley, G. R. Luckhurst and
C. P. Stockley, Proc. R. Soc. Lond. A, 1982, 381,
117. 3 R. Saladino, Tesi di Laurea, Università
della Calabria, A. A. 1993/94. 4 G. Celebre, M.
Longeri, in Encyclopedia of NMR , Eds D. M.
Grant and R. K. Harris, Wiley, New York,
1996. 5 T. Strassner, Can. J. Chem., 1997, 75,
1011. 6 P. Diehl in Encyclopedia of NMR ,
Eds D. M. Grant and R. K. Harris, Wiley, New
York, 1996. 7 S. A. Katsyuba, J. Grunenberg, R.
Schmutzler, J. Mol. Struct., 2001, 559, 315.