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Pressure dependence of NMR spectroscopy for studying intermolecular interactions 1H NMR in organic solvents involving the nitroxide radical
VoIume 61, number 2
PRESSURE
CHEMICAL
DEPENDENCE
FOR STUDYING
OF NMR SPECTROSCOPY
INTERiMOLECULAR
lH NMR IN ORGANIC
15 February 1979
PHYSICS LJXTERS
SOLVENTS
INTERACTIONS.
INVOLVING
THE NITROXIDE
RADICAL
KmEND0 *, Y_ HAZAiiA, K. OKABAYASHI, I_ TONOIKE and K_ SUZUKI Dcparrmenc of Chemisrry. Facrdry of Sciezzcr and Engzizeerin~. Rirsumeikm Umkrsity.
Kitaku. K~oto.
Japan
Received 8 November 1978
Pressure effects on t H chemical shifts and rekaation rates in organic solvents containing the nitroxide radical have been observed. ‘H chemical shifts with pressure and pressure-broadenings for hydrogenbondingbetweenproton-donormolecules and the nitroxide radical have been observed.
There have been several NMR studies [l-5] on dynamic and static properties of organic .nolecules in the liquid state at high pressure_ These experiments on high-pressure NMR have shown that the technique provides a useful tool for studying the behavior of solvent molecules.. We are also interested in high pressure NMR methods to investigate chemical phenomena associated with intermolecular interactions, such as hydrogen bonding and charge-transfer interactions_ We have studied pressure effects on proton chemical shifts and rehtxation times in liquids containing paramagnetic substances, and in this letter, we describe preliminary rest&s of ‘H NMR chemical shifts with pressure and pressure-line-broaden&s for hydrogen-bonded protons [6] in protic solvent moIecuIes (protondonor molecules) in interaction with nitroxide radicals. Measurements were made at 60 MHz and 35°C up to 10: bar, with a Hitachi cw NMR (R-24) spectrometer- CyIohe_xane was used as the internal reference since the chemical-shift difference between the internal and external cydohexane protons was independent of pressure within experimental error. Resolution of proton signals was maintained at 2 Hz at half-height_ The accuracy in the shift is f 1 Hz The line shape of t H spectra is assumed to be Iorentzian. AU T?‘s were evahrated from signal widths at half-height. The high * Present address: Department of Physics, Faculty of Science, Kyoto L’niversity. Kyoto. Japan
336
pressure glass sample cell was prepared by the method of Yamada [7] _ The glass capillary was connected to a high-pressureunit. We first investigated the pressure effects on iH resonance positions in pure organic solvent molecules. The protons of protic solvent molecules (CH,OIH and CIFJ%J experienced down-field shifts with increasing pressure (density), while aprotic protons (CtI-&OH, C6t& and C6t&) were almost unaffected_ The results for methanol are in good accord with the down-field shifts obtained by Oldenziel and Trappeniers [43 _The down-field shifts of protic solvents are caused by hydrogen bonding [3,4] or chemical association [5] between diamagnetic solvent molecules which approach more closely with increasing density. Fig_ I shows the pressure dependence of proton chemical shifts in protic solvents mixed with the di-tert-butyl nitroxide radical (DTBN)_ The slope of the iH chemical shift-pressure plot for CHCI, changes from negative (down-BeId) to positive (up-field) with Increasing pressure as the concentration of the radical is increased_ This result is interpreted as follows: In DTBN solutions of protic substances at high pressure, we measure both down-tieId and up-field shifts at the same time; the down-field shift depends on the formation of a hydrogen bond or chemical association between sohent molecules, while the up-fie!d shift is explained in terms of fgrmstion of a hydrogen bond [6] between
Volume 61. number 2
CHEMICAL PHYSICS LETTERS
Fig. 1. Pressure dependence of proton chemical shifts of protic solvents referred to cycloheune protons. CHC13 ...QTBN 1 and 2 denote mole ratios (A’/iVe) of DTBN to CHC13 of 0.014 and 0.042, respectively. CH30H ___DTBN 3 and 4 denote mole mn,tios of DTBN to CH30H of 0.0071 and 0.021, respectively.
the protic substance and DTBN_ For this protic solventDTBN hydrogen-bond system it was shown [6] that addition of a slight amount of DTBN produced an up-field shift reflecting increased shielding of the proton caused by the electron-donor molecule DTBN. Therefore, increasing density at the higher concentration of the radical in protic solvents brings up-field shifts which are interpreted in terms of increased protom shielding (increase of negative spin density at the solvent proton induced by hydrogen bonding with DTBN) on formation of the hydrogen bond between the protic solvent molecule and DTBN_ We have also observed the pressure dependence of 1H relaxation rates in organic solvents involving DTBN Fig_ 2 shows the pressure-broadening of the IH spectrum for ClljCI, containing DTBN. We plot 1H relaxation rates versus pressure for DTBN solutions of organic solvent molecules in fig. 3, where the relaxation rates are compared with Q for the pure solvents versus pressure [S ] since the viscosity changes with pressure_ Thus, in the case of aprotic solvent-DTBN systems the plots of ln(No/NT2) versus Pand lnq
15 February 1979
Fig. 2. ’ H spectra for CHCIs contking 1OODbar.
DTBN at 1,800 and
7
3-oll-_-_lo3 0 200
400
600
800
low
P bar Fig. 3. Pressure dependence of the reiaxstion rates and viscosities of organic solvents.
337
Volume 61. number 2 verssusP for C&I6 solvent-DTBN
CHEMICAL
and C&OH
hydrogen-bond
are parallel_ For protic
governed
systems the slope of
moIecuIes.
the In (X&l&)
versus P plot is not panlIe
of the In 4 versus
P plot.
15 February
PHYSICS LETTERS
with that
These results for viscosity dependence of relaxation rates with pressure seem to be in good accord with viscosity dependence of rekrxation rates with tempemture [9]_ We expIain these results in the foliowing terms: In organic solven t-DTBN radical systems. the simplified equations [9] for the dominant rektxation rate may be expressed as
by the translational
For a protic solvent-DTBN
diffusion
1979
of solvent
hydrogen-bond
system,
we consider eq. (2)_ If it is assumed that r in eq_ (2)
does not change with pressure and temperature, then the reIaxation rate l/T2 depends on the dipoku brownian correlation time of the solvated complex, rc_ ln fig. 3 the slope of the In (N0fNT2) versus P plot for CHSOIJ and CIJCIS is not parallel with that of the Inn versus P plot. This shows that the relaxation rate versus pressure is dominated solvated complex
by the tumbling
of a
of the solvent with the DTBN
radical. for aprotic soIvent-radical interaction), and as
interaction
(no chemicaI The authors are much indebted to Dr. H. Yamada of Kobe University and Dr. Y_ Taniguchi of our Iaboratory for advice in making the high-pressure
for protic solvent-_mdicaI hydrogen-bond no:ation of ref_ [PI has been used.
systems. The
in the case of an aproric soivent-DTBN
system. the
translational
correlation
using a Stokes-Einstein 7d = (%dkT)a,
time rd is aIso easiiy obtained reIationship:
Qe(QH i- Qe) ~
(3)
where “II and Q= are the radii of solvent and radical
moIecuIcs approximated by rigid spheres, respectively: n is the viscosity cf the pure solvent. If eqs. (1) and (3) are combined, we obtain: l/JWz = C_i(w)llIT
,
where C is dependent
(4) on the coefficients
in eqs. (I)
varying function, it may be considered as a constant_ If it is assumed that d, ~~~ and Q~ in eqs_ (1) and (3) do not change with temperature and pressure, then l/T’ wiil depend and (3). Sincej(m)
on Tand
is a slowly
q_ in our experiment
the temperature
is main-
tained at 35”C_ In fig. 3, the plots of In (;V,/ATz) ‘cersus P and In g verslls P for aprotic soivents (C6H6 and CIi30H)
are paraIIeI_ We find that the mechanism
for the changes in both
33s
unit. We
express our appreciation to Professors T. Hashi and M_ hlatsuoka and their fellows at Kyoto University for helpful discussions.
l/T2 and 71with pressure is
References [I 1 TX. Bull and J. Jonas, J. Chem. Phys. 52 (1970) 4.553. 121 B.D. Boss and E.D. StejQal. J. Chem. Phys. 45 (1966) 81. [3] J-W_ Linonski, Nan-I. Liu arid J. Jonas_J. Magn. Reson. 23 (1976) 455. [41 J-G_ Otdenziei and NJ. Trappeniers. Pbysica 83A (1976) 16:. I51 H_ Yamada. T_ Ishiwara and T_ Kinugasa, J. _4m. Chem. Sot. 96 (1974) 1935. [6] I. hforishima. K. Endo and T. Yonaawa. J. Chem. Phys. 58 (1973) 3146; J. Am. Chem. Sot. 93 (1971) 2048; Chem. Phys. Letters 9 (1971) 143.203. [7] H_ Yamada. Rev_ Sci. Instr_ 45 (1974) 640. [S] 1nternatiol;al Critical TabIes of Numerical Data, Physics, Chemistry and TechnoIo,T, The NationaI Rcscarch Council of the US-A_ (McGnwv-HI& New York, 1929). f9j K_ Endo. L Morishima and T_ Yonezawa. J_ Chem_ Phys. 67 (1977) 4760; K_ Endo. B. Knuettel. I_ sforishima. T. Inubushi and T_ Yonezwn. Chem. Phys_ Letters 31 (1975) 387.
PRESSURE
CHEMICAL
DEPENDENCE
FOR STUDYING
OF NMR SPECTROSCOPY
INTERiMOLECULAR
lH NMR IN ORGANIC
15 February 1979
PHYSICS LJXTERS
SOLVENTS
INTERACTIONS.
INVOLVING
THE NITROXIDE
RADICAL
KmEND0 *, Y_ HAZAiiA, K. OKABAYASHI, I_ TONOIKE and K_ SUZUKI Dcparrmenc of Chemisrry. Facrdry of Sciezzcr and Engzizeerin~. Rirsumeikm Umkrsity.
Kitaku. K~oto.
Japan
Received 8 November 1978
Pressure effects on t H chemical shifts and rekaation rates in organic solvents containing the nitroxide radical have been observed. ‘H chemical shifts with pressure and pressure-broadenings for hydrogenbondingbetweenproton-donormolecules and the nitroxide radical have been observed.
There have been several NMR studies [l-5] on dynamic and static properties of organic .nolecules in the liquid state at high pressure_ These experiments on high-pressure NMR have shown that the technique provides a useful tool for studying the behavior of solvent molecules.. We are also interested in high pressure NMR methods to investigate chemical phenomena associated with intermolecular interactions, such as hydrogen bonding and charge-transfer interactions_ We have studied pressure effects on proton chemical shifts and rehtxation times in liquids containing paramagnetic substances, and in this letter, we describe preliminary rest&s of ‘H NMR chemical shifts with pressure and pressure-line-broaden&s for hydrogen-bonded protons [6] in protic solvent moIecuIes (protondonor molecules) in interaction with nitroxide radicals. Measurements were made at 60 MHz and 35°C up to 10: bar, with a Hitachi cw NMR (R-24) spectrometer- CyIohe_xane was used as the internal reference since the chemical-shift difference between the internal and external cydohexane protons was independent of pressure within experimental error. Resolution of proton signals was maintained at 2 Hz at half-height_ The accuracy in the shift is f 1 Hz The line shape of t H spectra is assumed to be Iorentzian. AU T?‘s were evahrated from signal widths at half-height. The high * Present address: Department of Physics, Faculty of Science, Kyoto L’niversity. Kyoto. Japan
336
pressure glass sample cell was prepared by the method of Yamada [7] _ The glass capillary was connected to a high-pressureunit. We first investigated the pressure effects on iH resonance positions in pure organic solvent molecules. The protons of protic solvent molecules (CH,OIH and CIFJ%J experienced down-field shifts with increasing pressure (density), while aprotic protons (CtI-&OH, C6t& and C6t&) were almost unaffected_ The results for methanol are in good accord with the down-field shifts obtained by Oldenziel and Trappeniers [43 _The down-field shifts of protic solvents are caused by hydrogen bonding [3,4] or chemical association [5] between diamagnetic solvent molecules which approach more closely with increasing density. Fig_ I shows the pressure dependence of proton chemical shifts in protic solvents mixed with the di-tert-butyl nitroxide radical (DTBN)_ The slope of the iH chemical shift-pressure plot for CHCI, changes from negative (down-BeId) to positive (up-field) with Increasing pressure as the concentration of the radical is increased_ This result is interpreted as follows: In DTBN solutions of protic substances at high pressure, we measure both down-tieId and up-field shifts at the same time; the down-field shift depends on the formation of a hydrogen bond or chemical association between sohent molecules, while the up-fie!d shift is explained in terms of fgrmstion of a hydrogen bond [6] between
Volume 61. number 2
CHEMICAL PHYSICS LETTERS
Fig. 1. Pressure dependence of proton chemical shifts of protic solvents referred to cycloheune protons. CHC13 ...QTBN 1 and 2 denote mole ratios (A’/iVe) of DTBN to CHC13 of 0.014 and 0.042, respectively. CH30H ___DTBN 3 and 4 denote mole mn,tios of DTBN to CH30H of 0.0071 and 0.021, respectively.
the protic substance and DTBN_ For this protic solventDTBN hydrogen-bond system it was shown [6] that addition of a slight amount of DTBN produced an up-field shift reflecting increased shielding of the proton caused by the electron-donor molecule DTBN. Therefore, increasing density at the higher concentration of the radical in protic solvents brings up-field shifts which are interpreted in terms of increased protom shielding (increase of negative spin density at the solvent proton induced by hydrogen bonding with DTBN) on formation of the hydrogen bond between the protic solvent molecule and DTBN_ We have also observed the pressure dependence of 1H relaxation rates in organic solvents involving DTBN Fig_ 2 shows the pressure-broadening of the IH spectrum for ClljCI, containing DTBN. We plot 1H relaxation rates versus pressure for DTBN solutions of organic solvent molecules in fig. 3, where the relaxation rates are compared with Q for the pure solvents versus pressure [S ] since the viscosity changes with pressure_ Thus, in the case of aprotic solvent-DTBN systems the plots of ln(No/NT2) versus Pand lnq
15 February 1979
Fig. 2. ’ H spectra for CHCIs contking 1OODbar.
DTBN at 1,800 and
7
3-oll-_-_lo3 0 200
400
600
800
low
P bar Fig. 3. Pressure dependence of the reiaxstion rates and viscosities of organic solvents.
337
Volume 61. number 2 verssusP for C&I6 solvent-DTBN
CHEMICAL
and C&OH
hydrogen-bond
are parallel_ For protic
governed
systems the slope of
moIecuIes.
the In (X&l&)
versus P plot is not panlIe
of the In 4 versus
P plot.
15 February
PHYSICS LETTERS
with that
These results for viscosity dependence of relaxation rates with pressure seem to be in good accord with viscosity dependence of rekrxation rates with tempemture [9]_ We expIain these results in the foliowing terms: In organic solven t-DTBN radical systems. the simplified equations [9] for the dominant rektxation rate may be expressed as
by the translational
For a protic solvent-DTBN
diffusion
1979
of solvent
hydrogen-bond
system,
we consider eq. (2)_ If it is assumed that r in eq_ (2)
does not change with pressure and temperature, then the reIaxation rate l/T2 depends on the dipoku brownian correlation time of the solvated complex, rc_ ln fig. 3 the slope of the In (N0fNT2) versus P plot for CHSOIJ and CIJCIS is not parallel with that of the Inn versus P plot. This shows that the relaxation rate versus pressure is dominated solvated complex
by the tumbling
of a
of the solvent with the DTBN
radical. for aprotic soIvent-radical interaction), and as
interaction
(no chemicaI The authors are much indebted to Dr. H. Yamada of Kobe University and Dr. Y_ Taniguchi of our Iaboratory for advice in making the high-pressure
for protic solvent-_mdicaI hydrogen-bond no:ation of ref_ [PI has been used.
systems. The
in the case of an aproric soivent-DTBN
system. the
translational
correlation
using a Stokes-Einstein 7d = (%dkT)a,
time rd is aIso easiiy obtained reIationship:
Qe(QH i- Qe) ~
(3)
where “II and Q= are the radii of solvent and radical
moIecuIcs approximated by rigid spheres, respectively: n is the viscosity cf the pure solvent. If eqs. (1) and (3) are combined, we obtain: l/JWz = C_i(w)llIT
,
where C is dependent
(4) on the coefficients
in eqs. (I)
varying function, it may be considered as a constant_ If it is assumed that d, ~~~ and Q~ in eqs_ (1) and (3) do not change with temperature and pressure, then l/T’ wiil depend and (3). Sincej(m)
on Tand
is a slowly
q_ in our experiment
the temperature
is main-
tained at 35”C_ In fig. 3, the plots of In (;V,/ATz) ‘cersus P and In g verslls P for aprotic soivents (C6H6 and CIi30H)
are paraIIeI_ We find that the mechanism
for the changes in both
33s
unit. We
express our appreciation to Professors T. Hashi and M_ hlatsuoka and their fellows at Kyoto University for helpful discussions.
l/T2 and 71with pressure is
References [I 1 TX. Bull and J. Jonas, J. Chem. Phys. 52 (1970) 4.553. 121 B.D. Boss and E.D. StejQal. J. Chem. Phys. 45 (1966) 81. [3] J-W_ Linonski, Nan-I. Liu arid J. Jonas_J. Magn. Reson. 23 (1976) 455. [41 J-G_ Otdenziei and NJ. Trappeniers. Pbysica 83A (1976) 16:. I51 H_ Yamada. T_ Ishiwara and T_ Kinugasa, J. _4m. Chem. Sot. 96 (1974) 1935. [6] I. hforishima. K. Endo and T. Yonaawa. J. Chem. Phys. 58 (1973) 3146; J. Am. Chem. Sot. 93 (1971) 2048; Chem. Phys. Letters 9 (1971) 143.203. [7] H_ Yamada. Rev_ Sci. Instr_ 45 (1974) 640. [S] 1nternatiol;al Critical TabIes of Numerical Data, Physics, Chemistry and TechnoIo,T, The NationaI Rcscarch Council of the US-A_ (McGnwv-HI& New York, 1929). f9j K_ Endo. L Morishima and T_ Yonezawa. J_ Chem_ Phys. 67 (1977) 4760; K_ Endo. B. Knuettel. I_ sforishima. T. Inubushi and T_ Yonezwn. Chem. Phys_ Letters 31 (1975) 387.