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Quadrupole resonance of 14N and 2D in glycine, a model compound
Vclume 41. number 3
QUADRUPOLE
1 August I976
CHEMKAL. PHYSICS LETTERS
RESONANCE
OF 14N AND *D IN GLYCINE, A MODEL COMPOUND”
D.T. EDMONDS and C.P. SUMMERS are&on
Laboratory.
Departmenr of Physics, University of Oxford, Oxford.
UK
Received 31 March 1976
Using doubIe nuclear resonance techniques (DRLC) the quadrupole resonances spectrum of 14N is obtained in both protonated and deuterated glycine crystallized in each of three crystalline forms. In the deuterated samples the quadrupole resonance spectrum is obtained for each of the three amino deuterons again in each of the three crystalline forms. It is suggested that this system is ideal for a comparison between theoretical calculation and experiment measurement of molecular electric fIeId gradients.
1. Introduction Double resonance methods have increased the sensitivity of nuclear magnetic resonance (NMR) such that experiments performed in a routine manner today would have been quite out of the question only a few years ago. As a result, the applicability of NMR has grown, until it is becoming the most used analytical technique in many fields of chemistry and biology. The application of double resonance techniques, and il particular double resonance by level crossing (DRLC), has wrought a similar change in nuclear quadrupole resonance (NQR) so that it is possible iu many solids to detect with high sensitivit the NQR of such light nuclei as 2D, “B, “B, 14N, y7 0 23Na, =Mg and 27Al, many of which were undete:tabb in powder samples only a few years ago. NQR measures the electronic structure cf a molecule much more directly than does NMR in that it yields the magnitude and symmetry of the electric field gradient at the nuclear site. Furthermore, the electric rield adient, being essentially an average of ((32* - 12)/ F ) over the molecular wavefunctions, is a single electron function involving only ground state wavefunctions and SO should be much more easily calculated than for instance the chemical shift or the con-
tact shift in NMR. Another advantage of NQR is that, as we shall see, it is a very sensitive probe of the hydrogen bond, again because it directly measures the electric field gradient that gives rise to the bond. For all these reasons it would seem that, now that the experimental limitations have been overcome, NQR could become for solids what NMR has proved to be for liquids; a very widely used analytical tocl. In view of this, the amount of theoretical work devoted to the interpretation of NQR is disappointing. No doubt part of the reason is that until very recently it was possible to obtain experimental data easily only for heavy nuclei surrounded by many electrons for which a reliable calculation of the electric field gradient would be currently impossible. In this paper we seek to improve this situation very slightly by presenting a comprehensive experimental investigation of the NQR of 2D and 14N in glycine chosen as a relatively simple model compound for which calculations may prove possible. Glycine is chosen as a model compound for three reasons. In the first instant% glycine is important in its own right as the simplest amino acid and as an example of the terminal amino group of all amino acids. Secondly it is possrble to probe the electric field gradient of the amino group both at the niirogeell site and, by deuteration,
* This work was supported in part by the Science Research Council.
482
at the three individual
deuteron
sites.
Finally glycine crystallizes in three crystalline forms called o, P, and 7 in which the molecular configura-
VoXume41, number 3
CHEMICAL PHYSICS LE-iTERS
1 August 1976
tion is almost identical but the hydrogen bonding between molecules changes so that it is possibie to attempt to separate intra- and intermolecular effects upon the elecfric field gradients.
We have recently described the technique of DRLC in some detail [I ,2] so that we will not repeat it here. Both 14N and 2D have spin 1 and there are three detectable lines in the NQR spectrum given in the conventional notation and in order of increasing frequency by vo, v_ and v, where P&= ~~~2~4/~)~~ k 11/3) f
vo=v+-v_==, '@qQlhh. e2qQ/h
is the quadrupole coupling constant which for a given nucleus with nuclear q~ad~poIe moment Q measures the magnitude of the principal component of the electric field gradient. q is the anisotropy constant and reflects the asymmetry of the electric field gradient about the direction of the principai component such that, for example, ?I = 0 indicates axial symmetry. In table 1 we present the data obtained from 14N NQR studies of the three crystalline forms of glycine both protonated and deuterated. The results for glytine ethyl ester hydrochloride are presented for comparison. We have previously [3] reported the result for a+glycine and have also reForted [4] an investigation of the interpretation of such NQR data on 14N in a tetrahedral environment using *thesemi-emptical m&hod of Townes and Dailey [S] . To our knowledge there has been no published attempt to calculate the eiectric field gradient at such a nitrogen site. X-ray [6] and neutron [7] diffraction studies of cr-glycine and X-ray studies of fl-glycine [8] and y-glycine [9] reveal that the molecular geometry is almost identical in the three crystalhe forms with the two carbon atoms, the oxygen atoms and the nitrogen atom being very nearly planar. Tn the cy,p and y forms the N-C bond lengths are 1.491 4 O.Oi , I .484 f 0.0 15 and 1.474 + 0.01 A whi!st the disttm_ceof the n;‘rog% nucleus from the nearest carboxyl oxygen nucleus is 2.687 f 0.01,2.701 rt 0.015 and 2.690 f 0.01 A respec-
b 2: s
b 8 .
PI
N
2.
5
m
z i
p
’
483
CHEbWXL PHYSI[CSLETTERS
Volume 4 1, number 3 Tsblti 2 be fRquen&.s of&e swcw
1 August 1976
tie:; in the nuclearquadrupoleresonancespectrumof 2D and the deduced quadrupole coupling
cons*ants FOiTlUla
site
4kIiZ)
v_(kNz)
e4Qlh
N+D3’CH2 *COO-
N+Ds
107.420.2 120.720.2 140.2+0.2
104.4*0.2 I X7.1+0.2 137.4+0.2
l41.2+-0.3 158.5iO.3 185.1*0.3
0.0431-0.006 0.045-+0.005 0.030+0.004
103.0+0.5 N?D3*CHZ *COO-
N”Ds
121.0-*0.5
100.5+0.5 115.o+o.s
135.710.7 157.3+0.7
0.037”0.015 0.076~0.013
144.5t0.5
141.5~0.5
190.6+0.7
0.031~0.010
I)
&Hz)
N+D3 - CD2 -COO--
N-3
11 l.OI-05 121.0+0.5 132.5+0.5
107.0+0.5 117.5to.s 129.So.5
145.3+0.? 1.59.0+0.7 174.7*0.7
0.055+-0.014 0.044~0.013 0.03410.011
N+ffs-CD2 *COO-
CD,
132.2kO.S 129.6cO.5
124.750.5 126.3+&S
171.350.7 170.6kO.7
0.088~0.015 0.039=0.015
0
GLYCINE
(d
=
L-ALANINE
p
L-TYROSINE
180
160
. +.“‘@
1.03
Fig. 1. The quadrupole coupling constant e’qQ/h
e z *“m”““” 1.04
1-05
for ‘D sites in the amino group of deuterated &cine,
1.06
L-z&nine and L-tyrosine
plotted against the N+-D bond distmce as measured by neutron diffraction. tively.
The differences
in the electric
field gradient
re-
vealed by NQR must reside in the changes in proton positions due to different intermolecular hydrogen bonding. The reductions of 23 kHz, 23 kHz and 20 kHz in the 141Vquadrupole coupiing constant for the a, p and y forms on deuteration show a marked consistency and are in agreement with results on other amSo acids we have measured [I 0] . 484
In table 2 we present the results obtained from the NQR of *D in these same crysta!s. On deuteration of +Jzeamino group six lines are observed which occur in pairs and which may be positively assigned into v+, V_ pairs by analysing the behaviour of the lines on the application of a small (5 gauss) constant magnetic field to the sample during the irradiation phase. In this manner the electric field gradient is measured separately
Volume 41, number 3
CHEhWXL
PHYSICS LETTERS
each deuterium site. Deuterium has a single spherically symmetric Is electron whose excited states are at high energy so that the electric field gradient arises largely from the charges on adjacent atoms. We would expct the quadrupole coupling constant to be a sensitive function of e-D bond distance. That this is so is reveaied in fig. t where e2qQ/h is plotted against Nf--D bond length for glycine, Lalanine [lO,ll) and L-tyrosine [lo, 121. The first reference relates to the measure-) ment of e*qQ/h and the second to high precision neutron diffraction. In each case it is assumed that the higher value e2qQ/tt is to be associated with the sm’aller v&c of NC-D bond length. It is a pity that such high precision neutron diffraction data do not exist for many more of the amino acids including the 0 and 7 forms of glycine. Again we know of no attempt to calculate the electric field gradient that a deuteron would experience in such an environment. at
L August 1976
References [1] D-T. Edmonds. M-3. Hunt, A.L. Mackay and 6.P. Summers, in: Advances in nuclear quadrupole resonance, Vol. 1 (Heyden, London, 1974) p. 145. [2] D.T. Edmonds, Proceedings of the 18th Ampere Congress, Nottingham (1974) p. 13; Pure Appi. Chem. 40 (1974; 193. [ 31 D.T. Edmonds and P-A. Speight, J. Magn. Reson. 6 (1972) 265. ]4] D-1‘. Edmonds, M.J. Hunt and A-5. Mackay, I. Magn. Resort. 9 (1973) 66. [Sl B.P. Dailey and C.H. Townes, J. Citem. Wys. 23 (1955) 118. [6] R.E. Marsh, Acta Cryst. 11 (1958) 654. [7] P.G. Ionsson and A. Kvick, Acta C_ryst. B28 (1972) 1827. [8] Y. lit&a, Acta. Cryst. 13 (1960) 35. (91 Y. Iitaka, Acta Cryst. 14 (1961) I. [lo] M.J. Hunt and A.L. Mackay, J. Magn. Reson. 1S (1974) 402. [ 111 M.S. Lehmann, T-F. Koetzle and W.C. Hamilton, J. Am. Chem.Sot. 94 (1972) 2657. [12] MN. Frey, T.F. Koetzle, MS. Lehmann and1V.C. Hamilton, J. Chem. Phys. 58 (1973) 2547.
485
QUADRUPOLE
1 August I976
CHEMKAL. PHYSICS LETTERS
RESONANCE
OF 14N AND *D IN GLYCINE, A MODEL COMPOUND”
D.T. EDMONDS and C.P. SUMMERS are&on
Laboratory.
Departmenr of Physics, University of Oxford, Oxford.
UK
Received 31 March 1976
Using doubIe nuclear resonance techniques (DRLC) the quadrupole resonances spectrum of 14N is obtained in both protonated and deuterated glycine crystallized in each of three crystalline forms. In the deuterated samples the quadrupole resonance spectrum is obtained for each of the three amino deuterons again in each of the three crystalline forms. It is suggested that this system is ideal for a comparison between theoretical calculation and experiment measurement of molecular electric fIeId gradients.
1. Introduction Double resonance methods have increased the sensitivity of nuclear magnetic resonance (NMR) such that experiments performed in a routine manner today would have been quite out of the question only a few years ago. As a result, the applicability of NMR has grown, until it is becoming the most used analytical technique in many fields of chemistry and biology. The application of double resonance techniques, and il particular double resonance by level crossing (DRLC), has wrought a similar change in nuclear quadrupole resonance (NQR) so that it is possible iu many solids to detect with high sensitivit the NQR of such light nuclei as 2D, “B, “B, 14N, y7 0 23Na, =Mg and 27Al, many of which were undete:tabb in powder samples only a few years ago. NQR measures the electronic structure cf a molecule much more directly than does NMR in that it yields the magnitude and symmetry of the electric field gradient at the nuclear site. Furthermore, the electric rield adient, being essentially an average of ((32* - 12)/ F ) over the molecular wavefunctions, is a single electron function involving only ground state wavefunctions and SO should be much more easily calculated than for instance the chemical shift or the con-
tact shift in NMR. Another advantage of NQR is that, as we shall see, it is a very sensitive probe of the hydrogen bond, again because it directly measures the electric field gradient that gives rise to the bond. For all these reasons it would seem that, now that the experimental limitations have been overcome, NQR could become for solids what NMR has proved to be for liquids; a very widely used analytical tocl. In view of this, the amount of theoretical work devoted to the interpretation of NQR is disappointing. No doubt part of the reason is that until very recently it was possible to obtain experimental data easily only for heavy nuclei surrounded by many electrons for which a reliable calculation of the electric field gradient would be currently impossible. In this paper we seek to improve this situation very slightly by presenting a comprehensive experimental investigation of the NQR of 2D and 14N in glycine chosen as a relatively simple model compound for which calculations may prove possible. Glycine is chosen as a model compound for three reasons. In the first instant% glycine is important in its own right as the simplest amino acid and as an example of the terminal amino group of all amino acids. Secondly it is possrble to probe the electric field gradient of the amino group both at the niirogeell site and, by deuteration,
* This work was supported in part by the Science Research Council.
482
at the three individual
deuteron
sites.
Finally glycine crystallizes in three crystalline forms called o, P, and 7 in which the molecular configura-
VoXume41, number 3
CHEMICAL PHYSICS LE-iTERS
1 August 1976
tion is almost identical but the hydrogen bonding between molecules changes so that it is possibie to attempt to separate intra- and intermolecular effects upon the elecfric field gradients.
We have recently described the technique of DRLC in some detail [I ,2] so that we will not repeat it here. Both 14N and 2D have spin 1 and there are three detectable lines in the NQR spectrum given in the conventional notation and in order of increasing frequency by vo, v_ and v, where P&= ~~~2~4/~)~~ k 11/3) f
vo=v+-v_==, '@qQlhh. e2qQ/h
is the quadrupole coupling constant which for a given nucleus with nuclear q~ad~poIe moment Q measures the magnitude of the principal component of the electric field gradient. q is the anisotropy constant and reflects the asymmetry of the electric field gradient about the direction of the principai component such that, for example, ?I = 0 indicates axial symmetry. In table 1 we present the data obtained from 14N NQR studies of the three crystalline forms of glycine both protonated and deuterated. The results for glytine ethyl ester hydrochloride are presented for comparison. We have previously [3] reported the result for a+glycine and have also reForted [4] an investigation of the interpretation of such NQR data on 14N in a tetrahedral environment using *thesemi-emptical m&hod of Townes and Dailey [S] . To our knowledge there has been no published attempt to calculate the eiectric field gradient at such a nitrogen site. X-ray [6] and neutron [7] diffraction studies of cr-glycine and X-ray studies of fl-glycine [8] and y-glycine [9] reveal that the molecular geometry is almost identical in the three crystalhe forms with the two carbon atoms, the oxygen atoms and the nitrogen atom being very nearly planar. Tn the cy,p and y forms the N-C bond lengths are 1.491 4 O.Oi , I .484 f 0.0 15 and 1.474 + 0.01 A whi!st the disttm_ceof the n;‘rog% nucleus from the nearest carboxyl oxygen nucleus is 2.687 f 0.01,2.701 rt 0.015 and 2.690 f 0.01 A respec-
b 2: s
b 8 .
PI
N
2.
5
m
z i
p
’
483
CHEbWXL PHYSI[CSLETTERS
Volume 4 1, number 3 Tsblti 2 be fRquen&.s of&e swcw
1 August 1976
tie:; in the nuclearquadrupoleresonancespectrumof 2D and the deduced quadrupole coupling
cons*ants FOiTlUla
site
4kIiZ)
v_(kNz)
e4Qlh
N+D3’CH2 *COO-
N+Ds
107.420.2 120.720.2 140.2+0.2
104.4*0.2 I X7.1+0.2 137.4+0.2
l41.2+-0.3 158.5iO.3 185.1*0.3
0.0431-0.006 0.045-+0.005 0.030+0.004
103.0+0.5 N?D3*CHZ *COO-
N”Ds
121.0-*0.5
100.5+0.5 115.o+o.s
135.710.7 157.3+0.7
0.037”0.015 0.076~0.013
144.5t0.5
141.5~0.5
190.6+0.7
0.031~0.010
I)
&Hz)
N+D3 - CD2 -COO--
N-3
11 l.OI-05 121.0+0.5 132.5+0.5
107.0+0.5 117.5to.s 129.So.5
145.3+0.? 1.59.0+0.7 174.7*0.7
0.055+-0.014 0.044~0.013 0.03410.011
N+ffs-CD2 *COO-
CD,
132.2kO.S 129.6cO.5
124.750.5 126.3+&S
171.350.7 170.6kO.7
0.088~0.015 0.039=0.015
0
GLYCINE
(d
=
L-ALANINE
p
L-TYROSINE
180
160
. +.“‘@
1.03
Fig. 1. The quadrupole coupling constant e’qQ/h
e z *“m”““” 1.04
1-05
for ‘D sites in the amino group of deuterated &cine,
1.06
L-z&nine and L-tyrosine
plotted against the N+-D bond distmce as measured by neutron diffraction. tively.
The differences
in the electric
field gradient
re-
vealed by NQR must reside in the changes in proton positions due to different intermolecular hydrogen bonding. The reductions of 23 kHz, 23 kHz and 20 kHz in the 141Vquadrupole coupiing constant for the a, p and y forms on deuteration show a marked consistency and are in agreement with results on other amSo acids we have measured [I 0] . 484
In table 2 we present the results obtained from the NQR of *D in these same crysta!s. On deuteration of +Jzeamino group six lines are observed which occur in pairs and which may be positively assigned into v+, V_ pairs by analysing the behaviour of the lines on the application of a small (5 gauss) constant magnetic field to the sample during the irradiation phase. In this manner the electric field gradient is measured separately
Volume 41, number 3
CHEhWXL
PHYSICS LETTERS
each deuterium site. Deuterium has a single spherically symmetric Is electron whose excited states are at high energy so that the electric field gradient arises largely from the charges on adjacent atoms. We would expct the quadrupole coupling constant to be a sensitive function of e-D bond distance. That this is so is reveaied in fig. t where e2qQ/h is plotted against Nf--D bond length for glycine, Lalanine [lO,ll) and L-tyrosine [lo, 121. The first reference relates to the measure-) ment of e*qQ/h and the second to high precision neutron diffraction. In each case it is assumed that the higher value e2qQ/tt is to be associated with the sm’aller v&c of NC-D bond length. It is a pity that such high precision neutron diffraction data do not exist for many more of the amino acids including the 0 and 7 forms of glycine. Again we know of no attempt to calculate the electric field gradient that a deuteron would experience in such an environment. at
L August 1976
References [1] D-T. Edmonds. M-3. Hunt, A.L. Mackay and 6.P. Summers, in: Advances in nuclear quadrupole resonance, Vol. 1 (Heyden, London, 1974) p. 145. [2] D.T. Edmonds, Proceedings of the 18th Ampere Congress, Nottingham (1974) p. 13; Pure Appi. Chem. 40 (1974; 193. [ 31 D.T. Edmonds and P-A. Speight, J. Magn. Reson. 6 (1972) 265. ]4] D-1‘. Edmonds, M.J. Hunt and A-5. Mackay, I. Magn. Resort. 9 (1973) 66. [Sl B.P. Dailey and C.H. Townes, J. Citem. Wys. 23 (1955) 118. [6] R.E. Marsh, Acta Cryst. 11 (1958) 654. [7] P.G. Ionsson and A. Kvick, Acta C_ryst. B28 (1972) 1827. [8] Y. lit&a, Acta. Cryst. 13 (1960) 35. (91 Y. Iitaka, Acta Cryst. 14 (1961) I. [lo] M.J. Hunt and A.L. Mackay, J. Magn. Reson. 1S (1974) 402. [ 111 M.S. Lehmann, T-F. Koetzle and W.C. Hamilton, J. Am. Chem.Sot. 94 (1972) 2657. [12] MN. Frey, T.F. Koetzle, MS. Lehmann and1V.C. Hamilton, J. Chem. Phys. 58 (1973) 2547.
485