High Temperatures - High Pressures, 1977, volume 9, pages 519-520
Determination of the activation volume of the rotation of the dimethylamino group in /^/V-dimethylamides
Elmar Lang, Rudolf Rauchschwa I be, Hans-Dietrich Ludemann
Institut fur Biophysik und Physikalische Biochemie, Universitat Regensburg, F R G
Paper presented at the 15th Annual Meeting of the European High Pressure Research Group, Darmstadt, F R G , 28-31 March 1977
A recently developed high-pressure cell for use in a standard high-resolution n m r spectrometer is described here; the cell (figure 1) is a m o d i f i c a t i o n o f a design first p u b l i s h e d b y Y a m a d a ( 1 9 7 4 ) . It fits i n t o the standard 12 m m probe configuration o f a V a r i a n X L - 1 0 0 - 1 5 spectrometer. The n m r cell proper consists o f a carefully
annealed glass capillary w i t h 1 • 5 m m inner diameter and 7 m m outer diameter. T h e neck o f this capillary is glued w i t h e p o x y resin i n t o the bore o f a c o p p e r - b e r y l l i u m n i p p l e . T h e upper end o f the c a p i l l a r y is connected to a T e f l o n shrink hose c o n t a i n i n g the s o l u t i o n under investigation and serving as the pressure-transmitting m e d i u m . T h e field h o m o g e n e i t y o f the V a r i a n 15 i n magnet is sufficient for a resolution o f 1 H z or slightly better i n this design w i t h o u t spinning the probe. The cell has been operated successfully from + 150 to - 100 ° C . Careful selection o f the capillaries allows w o r k i n g pressures o f up to 2-5 kbar.
H i g h r e s o l u t i o n n m r at variable temperatures has become the most p r o m i n e n t m e t h o d for the d e t e r m i n a t i o n o f the energies o f activation o f simple c o n f o r m a t i o n a l rearrangements i n the l i q u i d phase. F r o m the coalescence temperature o f the t w o resonance signals, tc, and from the frequency difference at l o w temperatures, bv, energies and entropies o f activation can be obtained i n an almost routine fashion ( G u n t h e r 1973).
standard cone seal
Safety jacket
Teflon screw
copper-beryllium cap 3:
copper-beryllium ring
nmr tube
12 mm outer diameter 9 mm inner diameter
— glass stopper
— Teflon hose
copper-beryllium cell
copper-beryllium cone
copper-beryllium screw
epoxy resin
high-pressure glass cell 7 mm outer diameter 1 • 5 mm inner diameter J1 cm
Figure 1. High-pressure cell for use in standard high-resolution nmr spectrometers.
520 E Lang, R Rauchschwalbe, H-D Ludemann
A s an example, we studied the r o t a t i o n o f the d i m e t h y l a m i n o group i n 7V,iV-dimethyl- acetamide, i n N , N - d i m e t h y l f o r m a m i d e , and i n N , A ^ d i m e t h y l b e n z a m i d e , around the C - N b o n d . T h e spectral parameters f o u n d for these c o m p o u n d s are given i n table 1.
Inserting A Gt, G i b b s ' free energy, and kc, the inversion rate at fc, o b t a i n e d at the various pressures into A r r h e n i u s ' e q u a t i o n yields a f o r m u l a for the temperature dependence o f the inversion rate, k.
Table 1. Parameters of the proton signals used in the evaluation of the activation parameters.
The compounds were dissolved in 40% d6-acetone-20% tetramethylsilane.
Compound 'c (0C) bv (s~ l) AG* (kcal mol l)
0-05 2-0 0-05 2-0 0-05 2-0
kbar kbar kbar kbar kbar kbar A^dimethylformamide 125 134 16-3 17-3 20-7 21-2 Af,VV-dimethylacetamide 74 83 17-1 17-7 17-9 18-4 AfA^imethylbenzamide 13 19-5 11-3 10-3 15-0 15-4
p (kbar)
Figure 2. Semilogarithmic plot of the inversion rate k of the dimethylamino group for three NfA^dimethylamides as a function of pressure.
T h e results, p l o t t e d i n figure 2, were calculated for integral temperatures i n the v i c i n i t y o f the respective coalescence temperatures. R a i s i n g the pressure to 2 k b a r reduces the inversion rate i n all three amides by ~ 5 0 % . W i t h the k n o w l e d g e o f the slope o f the curves, the activation volumes A K * can be derived from
d ( l n k) _ AV^
dp RT '
T h o u g h A G * varies by ~ 3 0 % , the v a r i a t i o n o f A F * is considerably smaller. T h e change f r o m 8-5 c m3 m o l "1 for d i m e t h y l b e n z a m i d e to 10-3 c m3 mol""1 for d i m e t h y l - acetamide is almost w i t h i n the range o f experimental r e p r o d u c i b i l i t y . T h e v a r i a t i o n o f A G * w i t h the change o f the t h i r d substituent at the c a r b o n y l c a r b o n is e x p l a i n e d by the influence o f this substituent o n the double b o n d character o f the C - N b o n d ( E m s l e y et al 1966). T h e relative independence o f the activation v o l u m e f r o m this substituent allows, i n our o p i n i o n , a m o r e geometric or steric e x p l a n a t i o n o f A K * . References
Emsley J W, Feeney J , Sutcliffe L H , 1966 High Resolution Nuclear Magnetic Resonance-Spectroscopy Vol 1 (Oxford: Pergamon Press) p 556
Gunther H , 1973 NMR-Spektroskopie (Stuttgart: Georg Thieme Verlag) p 239 Yamada H , 1974 Rev. ScL Instrum. 45 690
© 1977 a Pion publication printed in Great Britain
