Nuclear Quadrupole Resonance, Applications

NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS 1653

Acids Studied Using NMR; Proteins Studied Using NMR Spectroscopy; Structural Chemistry Using NMR Spectroscopy, Organic Molecules; Structural Chemistry Using NMR Spectroscopy, Peptides; Structural Chemistry Using NMR Spectroscopy, Pharmaceuticals; Two-Dimensional NMR Methods.

Further reading Abragam A (1989) Time Reversal, an Autobiography, p 164. Oxford: Oxford University Press. Balaram P, Bothner-By AA and Dadok JJ (1972). Journal of the American Chemical Society 94: 4015. Boulat B and Bodenhausen G (1992). Journal of Chemical Physics 97: 6040. Chiarpin E, Pelupessy P, Cutting B, Eykyn TR and Bodenhausen G (1998) Ang Chemie (in the press). Dalvit C and Bodenhausen G (1990) Advances in Magnetic Resonance 14: 1. Ernst RR, Bodenhausen G and Wokaun A (1987) Principles of Nuclear Magnetic Resonance in One and Two-dimensions. Oxford: Clarendon Press. Karthik G (1999) Journal of Chemical Physics 110: 4992.

Kumar Anil, Grace RCR and Madhu PK (1998) Progress in NMR Spectroscopy (in the press). Levy GC, Lichter R and Nelson GL (1980) Carbon-13 Nuclear Magnetic Resonance for Organic Chemists. NewYork: Wiley Interscience. Nageswara Rao BDN (1970) Nuclear spin relaxation by double resonance. Advances in Magnetic Resonance 4: 271. Neuhaus D and Williamson M (1989) The Nuclear Overhauser Effect in Structural and Conformational Analysis. NewYork: VCH. Noggle JH and Schirmer RE (1971) The Nuclear Overhauser Effect – Chemical Application. London: Academic Press. Overhauser AW (1996) In: Grant DM and Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance, Vol 1, p 513. Chichester: Wiley. Sanders JKM and Hunter BK (1987) Modern NMR Spectroscopy. Oxford: Oxford University Press. Slichter CP (1978) Principles of Magnetic Resonance. NewYork: Springer-Verlag. Wüthrich K (1976) NMR in Biological Research: Peptides and Proteins. Amsterdam: North Holland. Wüthrich K (1986) NMR of Proteins and Nucleic Acids. NewYork: Wiley.

Nuclear Quadrupole Resonance, Applications Oleg Kh Poleshchuk, Tomsk Pedagogical University, Russia Jolanta N Latosi ska, Adam Mickiewicz University, Poznan, Poland

MAGNETIC RESONANCE Applications

Copyright © 1999 Academic Press

The NQR frequencies for the various nuclei vary from several kHz up to 1000 MHz. Their values depend on quadrupole moments of the nucleus, the valence electrons state and the type of chemical bond in which the studied atom participates. Using the NQR frequencies, the quadrupole coupling constant (QCC) and asymmetry parameter ( η) can be calculated with either exact or approximate equations, according to the spin of the nuclei. For a polyvalent atom, the NQR frequencies depend on the coordination number and hybridization. The NQR frequency shifts for a single-valent atom in different environments can be classified as follows: 1. The greatest shifts of the NQR frequencies are determined by the valence electron state of the neighbouring atoms and can reach 1200–1500%. The changes of the NQR frequencies depend on ionic character, and for example for chlorine the

lowest frequency corresponds to a chloride ion, whereas the highest corresponds to the chlorine atom in ClF3. 2. Changes within the limits of one type of chemical bond (with the same kind of atom) can reach 40– 50%. Thus the frequency changes of C–Cl bonds vary from 29 to 44 MHz. 3. The range of possible shifts of the NQR frequencies is reduced to 10–20% when only one class of compound is studied. In this case the shifts are determined by the surroundings and donor– acceptor properties of the substituents. For example, for halogen substituted benzenes, the changes of the frequencies for the C–Cl bonds are about 9% for C–Br bonds ∼ 12% and for C– I bonds ∼ 18%. 4. The changes of NQR frequencies caused by the occurrence of intramolecular and intermolecular interactions lie within the limits of 3–40%.

1654 NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS

5. The shifts of NQR frequencies caused by crystal field effects in molecular crystals reach a maximum of 1.5–2%. However within a series of similar compounds this effect, as a rule, does not exceed 0.3%. Such classification of NQR frequency shifts allows the determination of structural non-equivalencies from NQR spectra. Understanding how the frequency shifts depend on chemical non-equivalence and how they are determined by distinctions in distribution of electronic density in a free molecule is very important. Moreover, crystallographical non-equivalences are observed, and are a result of distinct additional contributions of the crystal field to the electric field gradients at a nucleus. It is obvious that the division of structural non-equivalencies into molecular and crystal types is not relevant in coordination and ionic crystals, in which there are no individual molecules. NQR, being highly sensitive to subtle changes in electron density distribution, provides diverse information on the structural and chemical properties of compounds.

Molecular structure studies using NQR When applied to structural investigations, NQR spectra may prove an effective tool for the preliminary study of crystal structure in the absence of detailed X-ray data. Such parameters as spectroscopic shifts, multiplicity, spectroscopic splitting, resonance line width, the temperature dependence of resonance frequencies and relaxation rates all afford useful structural information and provide insight into the factors determining the formation of certain structural types. The violation of chemical equivalence of resonance atoms due to a change in chemical bonding, such as, for example, dimerization in group IIIA halogenides, leads to a significant splitting of the spectroscopic multiplet caused by a difference in the electronic structure of bridging and terminal atoms. Crystallographic structures

For crystallographically non-equivalent atoms the corresponding components of the electric field gradient (EFG) at the respective sites differ from each other in magnitude and direction due to the crystal field effect. This generally includes a contribution to the EFG of electrostatic forces between molecules, dispersion forces, intermolecular bonding and shortrange repulsion forces. Physically non-equivalent sites differ from each other only in the direction of

Table 1 The angles (°) between the directions of the C–Cl bonds and the crystal axes in 1,3,5-trichlorobenzene according to NQR and X-ray results

Crystal axes

Chlorine a position NQR

X-ray

NQR

X-ray

Cl-1

119.1

118.8

149.5

150

81.7

80.9

Cl-3

64.1

65.0

31.3

30.2

73.7

74.3

Cl-5

24.8

24.0

90.4

90.5

114.8

114.0

b

c NQR

X-ray

the EFG components, their magnitudes being identical. To distinguish between such sites, Zeeman analysis of the NQR spectrum is required. The intensities of spectroscopic lines are also important. They reflect the relative concentration of resonance nuclei at certain sites although one also has to take into account the transition probabilities and lifetimes of the energy states of the system investigated. The correspondence between the number and intensities of frequencies and the number of non-equivalent sites occupied by a resonant atom in a crystal lattice is very helpful in a preliminary structure study made with the use of NQR. NQR single-crystal Zeeman analysis can provide information about special point positions occupied by the quadrupole atoms. This Zeeman analysis determines the orientation of the EFG components with respect to the crystal axes, which essentially facilitates the most difficult and time-consuming stage of X-ray analysis. Table 1 gives a comparison of the angle values between the crystal axes and the EFG z-axis at the chlorine atoms in 1,3,5-trichlorobenzene (determined by the two methods). As one can see from Table 1 the angle values obtained by the two methods show rather good numerical agreement. To illustrate more completely the type of structural information that can be obtained by NQR spectroscopy we consider in more detail an NQR study of BiCl3, whose structure is known. Two chlorine atoms are involved in bridging to two other Bi atoms while another chlorine atom is involved in bridging to only one other Bi atom. The 35Cl and 209Bi NQR parameters of BiCl measured by 3 means of the single-crystal Zeeman method are listed in Table 2. The assignment of 35Cl resonances is unambiguous owing to direct correspondence of the lower frequency line of double intensity to the two Cl atoms which are crystallographically nearly equivalent. The asymmetry parameter for the chlorine atoms has been determined, using the zero-splitting cone. According to the results the BiCl3 crystal belongs to the Laue symmetry of D2h and therefore to the orthorhombic crystal class. The Cl(1) and Bi atoms

NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS 1655

Table 2

35

Cl and 209Bi NQR spectra of BiCl3

Transition frequencies (MHz) Isotope 35

Cl

T (K) 291

15.952(2)

–

–

–

25.132

37.362

51.776

19.173(1) 209

B

294

31.865

occupy special point positions in the lattice since only two non-equivalent directions of the corresponding EFG components have been detected for them, while four orientations have been found for the EFG components at the site of Cl(2,3) atoms. The number of observed 35Cl resonances suggests that the molecule possesses a mirror plane and therefore the centrosymmetric group Pnma. The lower frequency resonance of double intensity is then assigned to the chlorines out of the mirror plane while the higher frequency line corresponds to one lying in the mirror plane. The shorter (i.e. more covalent) bond length of the latter corresponds to the higher resonance frequency. Another example is provided by the 35Cl results for phosphorus pentachloride. In the gas phase this is known to be a trigonal bipyramid but the usual solid PCl5 is likewise known to be an ionic crystal, (PCl4)+(PCl6)−. In accordance with this, and the detailed crystal structure, there are four resonances at high frequency that correspond to the (PCl4)+ group and six at a lower frequency that correspond to the (PCl6)− group (Table 3). It has recently been observed that quenching of the vapour of PCl5 gives rise to a new metastable crystalline phase which can be preserved essentially indefinitely at low temperatures. The 35Cl spectrum of this has two low frequencies, corresponding to the axial chlorine atoms, and three identical higher frequencies, corresponding to the equatorial substituents, which is strong evidence that this new phase is the corresponding molecular solid. Another example is taken from the chemistry of antimony pentachloride. It is known that the molecule of antimony pentachloride at 210 K has a trigonal bipyramid structure, and at 77 K it is a dimer. In Table 3 the experimental 35Cl NQR frequencies and their assignment to equatorial and axial chlorine atoms are given. In the dimer molecule, bridging chlorine atoms have much lower NQR frequencies, as in dimer molecules of group IIIA compounds. From Table 3 it is also clear that in the monomer the NQR frequencies of equatorial chlorine atoms are greater than the axial ones.

e2Qqh−1 (MHz)

K (%)

30.960

43.1

Cl(1)

38.145

17.8

Cl(2)

318.900

55.5

Assignment

In the dimer, the ratio of 35Cl NQR frequencies is in agreement with the results of ab initio calculations. In dimers of transition elements, such as NbCl5 and TaCl5, the 35Cl NQR frequencies of equatorial chlorine atoms are higher than those of axial, whilst those of bridging chlorine atoms are higher, on average, than those of terminal chlorines. A similar inversion of NQR frequencies in dimers of transition and non-transition elements is explained by a significant multiplicity of the metal–halogen bonds and hence by electron transfer from p-valent orbitals of the halogen atoms to the vacant d-orbitals of the central atom. The fact that the difference between chemically non-equivalent atomic positions is readily revealed by NQR spectroscopic splitting may be utilized to identify geometric isomers. Octahedral complexes of tin tetrachloride, [SnCl4⋅L2] exist as either cis or trans isomers. In the cis isomers the axial and equatorial non-equivalence produces considerable splitting in the NQR spectra. In the trans complexes, all four chlorine atoms are chemically equivalent with identical electron density distribution. Splitting in the NQR spectra of these isomers arises therefore from the crystallographic non-equivalence of the chlorine positions. Indeed, the observed NQR splitting of two complexes (Table 4) provides evidence Table 3 35Cl quadrupole resonance frequencies for phosphorus and antimony pentachlorides

Compound

Q 35Cl (MHz)

Assignment

(PCl4)+(PCl6)–

28.395, 28.711, 29.027,

(PCl6)–

30.060, 30.457, 30.572

(PCl6)−

32.279, 32.384, 32.420

(PCl4)+

32.602

(PCl4)+

29.242, 29.274

Axial

33.751

Equatorial

PCl5 SbCl5 Sb2Cl10

30.18

Equatorial

27.85

Axial

27.76

Equatorial

30.18

Axial

18.76

Bridging

1656 NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS

Table 4 35Cl Quadrupole resonance frequencies and asymmetry parameters of some [SnCl4⋅2L] and [SbCl5⋅L] complexes

Complex

Q 35Cl (MHz)

K(%)

Assignment

[SnCl4⋅(NC5H5)2]

17.644 17.760

15.4

–

[SnCl4⋅(OPCl3)2]

19.030

11.7

Equatorial

19.794

11.1

Equatorial

21.132

2.3

24.399

11.3

25.821

2.5

Trans

26.119

4.7

Cis

[SbCl5⋅OPCl3]

[SbCl5⋅NCCCl3]

15.4

–

Axial

Cis

27.314

4.9

Cis

26.008

6.2

Cis

26.313

10.3

Cis

26.409

2.2

27.297

11.2

Trans Cis

for the cis configuration of [SnCl4⋅(OPCl3)2] and the trans configuration of [SnCl4⋅(NC5H5)2], which is confirmed by X-ray data. However, it is not always possible to assign equatorial and axial chlorine atoms solely on the basis of the splitting of the 35Cl NQR frequencies. For cis isomers, the ratio of the NQR frequencies of equatorial and axial chlorine atoms is fixed by several factors that determine the optimum crystal structure, among which is the influence of donor molecules, L is not a major contribution. In practically all structural investigations of complexes that exhibit this type of equatorial Sn–Cl bond it is usually noted that axial chlorines have lower relative NQR frequencies compared with equatorial atoms (Table 4). However, interpretation in structure terms is difficult because of the large values of the asymmetry parameters of chlorine atoms, which (Table 4) differ considerably for axial and equatorial atoms. On the other hand, donor ligands, L, influence the change in electron density of equatorial and axial Cl atoms and cause a relative lowering of frequencies of axial atoms in comparison with those of equatorial chlorines. Such a relation of frequencies in cis isomers is explained from the point of view of the ‘mutual ligand influence’ concept in non-transition element complexes. In cis complexes SnCl4L2 the interaction of –Sn–L bonds will be stronger with Sn–Cl bonds which are in a trans position, than with cis-Sn–Cl bonds. For these complexes, the mutual ligand influence establishes the greater trans effect, and leads to a redistribution of the electronic density on the chlorine atoms. In SbCl5⋅L complexes (Table 4) to assign axial and equatorial chlorine atoms signals, even

allowing for a knowledge of frequencies, splittings and asymmetry parameters, it is necessary in a number of cases to use the temperature dependences of NQR frequencies. In this case the NQR frequency is described by a square-law dependence: Q (T) = A + BT + CT 2. A positive sign of the C coefficient indicates that a NQR frequency arises from an axial chlorine atom. Thus it appears that in some complexes the frequency of an axial chlorine atom lies above those of equatorial atoms. Apparently, in these complexes the spatial influence of the donor molecules on the NQR frequencies of the equatorial chlorine atoms is dominant. The width of the NQR signal also provides structural information. In molecular crystals of high order and purity the line width is not much different from the value determined from the sum of the spin–lattice relaxation and spin–spin relaxation times. In the majority of inorganic compounds, the lines are, however, inhomogeneously broadened by lattice imperfections such as defects, vacancies, admixtures and dislocations, so that their widths are mainly determined by the crystal inhomogeneity. A systematic study of spectroscopic shifts and broadening produced by a continuous change of the relevant sources of that broadening is an effective approach to the investigation of problems concerned with the distribution of mixtures over a matrix, the nature of their interaction with the matrix, the mechanisms of disorder and the local order in vitreous compounds.

Studies of bonding using NQR spectra The Townes–Dailey approximation

The most widely used approach to provide a meaningful account of bonding trends within a series of related compounds is that formulated by Townes and Dailey for the interpretation of nuclear quadrupole interactions (NQI). The electric field gradient at a quadrupole nucleus (qzz) arises mainly from electrons of the same atom. To a first approximation, it is possible to consider that the internal electrons will form a closed environment with spherical symmetry and, consequently, do not contribute to the EFG. Actually, polarization of the internal electrons is taken into account through the Sternheimer antishielding factor (γ∞). However, if the comparison of NQI is only for the purpose of chemical interpretation and is not accompanied by discussion of their absolute meanings, the polarization of the internal electrons can be neglected. Among valent electrons, those that are on s orbitals with spherical symmetry do not

NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS 1657

contribute to the EFG and the main contribution is caused by p electrons; the contribution of d and f electrons is much less significant because of their greater distance from the nucleus and their smaller participation in hybridization. The quantitative consideration of the contributions to the EFG results in expressions of the following type, applied here to nuclei of chlorine in chloroorganic compounds:

where s2 and d2 are contributions from s and d orbitals to the hybridization of the chlorine atom bonding orbital, IB is the ionic character of the bond (the chlorine atom carries a partial negative charge) and qatCl is the gradient of p electron density on the chlorine atom. The valent orbitals can be represented by somewhat modified expressions in that some treatments include the three nuclear p orbital populations, and the axes x, y and z are usually defined so that the z-axis coincides with the bond direction of the considered halogen or with an axis of symmetry in the molecule. These approaches can be more convenient for discussing bonding and the contribution of lone pair electron orbitals. On the basis of the above interpretation NQI can be unequivocally represented in terms of the population of orbitals. Actually, NQR spectroscopy allows the determination, at best, of two parameters (e2Qqh−1 and K) and in many cases (I = ) only e2Qqh−1 can be obtained. However, the above-mentioned Equation [1] contains four parameters (s, d, IB, S) which cannot be determined from one or two experimental parameters. It is therefore necessary to include approximations, which neglect the d orbital participation in hybridization, and also, in some cases, p bonding and to consider that the s orbital hybridization is a small contribution and remains constant in a series of compounds. Thus changes of e2Qqh−1 are directly related to bond ionic character or p electron charge transfer in the case of a hydrogen bond. In the case of nitrogen, whose nucleus has a spin I = 1, the situation is more favourable, as the experiment allows the determination of both the nuclear quadrupole coupling constant and the asymmetry parameter, and it is possible to make conclusions about sp hybridization, if the molecular geometry is known. However, the meaning of nuclear NQI for 14N p electrons is not known with as much accuracy as for chlorine, bromine or iodine, but the estimation

of a 9–10 MHz contribution can be considered reliable, as it is based on the analysis of a large number of experimental data. An even more difficult situation is the case of the antimony atom, for which very little reliable NQI data exist. Donor–acceptor interactions

In addition to interpreting experimental NQI values in terms of orbital populations, a role is also played by structural, dynamic and simple contributions, which change the NQI such that their experimental meanings differ from those expected for a hypothetical molecule or complex in an isolated condition at rest. In the case of molecular complexes there is an additional contribution which results in NQI changes caused by a change in hybridization owing to a change in the donor and the acceptor molecule geometry. A typical example is given by MX 3 complexes, where M is a group IIIA or VB element and X is a halogen or methyl group. With complex formation, the X–M–X angle and appropriate hybridization change, and these result in changes in NQI interpretation even in the absence of any charge transfer. This complicates unequivocal interpretation of experimental NQR data. It is necessary to answer the question: are the shifts of the NQR frequencies caused by a transition from a pure, non-complexed mixture of initial substances to a complex, or because of a charge transfer or for other reasons? This rather important question depends on several factors. With a small charge transfer the NQR frequency shifts are defined mainly by crystal or solid-state effects, which are caused by distinct effects in the crystal environment of molecules as the result of the transition from individual components to the complex (crystal electrical field, intermolecular interactions, thermal movement); the complexation shifts can reach several hundred kHz (in the specific case of a resonance on a chlorine nucleus a shift of the order 200 kHz is typical). When the observable shifts have larger values (one to several MHz) they cannot be considered as caused by crystal effects and it is then possible with confidence to attribute them to electronic effects arising from a charge transfer. However, it is necessary to take into account other contributions, such as hybridization changes. The hybridization change accompanying deformation of a flat AlMe3 molecule to a pyramidal form formally results in an NQI change on the aluminium atom, even if there is no electron population change or change in ionic character of the bonding orbitals; thus, the shift of the NQR frequency in this case can be determined as having both a charge and a hybridization contribution.

1658 NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS

A more difficult situation exists when, in the free compounds, there are strong intermolecular interactions: the perturbation of these interactions by complex formation can result in an increased NQI in a complex which contradicts the usual, simple prediction of an NQI reduction upon transition to a complex. Such situations are met in complexes of mercury halogenides such as HgBr2⋅dioxan and HgI2⋅dioxan. More complete interpretation of the experimental results can be achieved if in a complex there is present a number of quadrupole nuclei; this allows a comparison of shifts for each of them. In addition, there is often useful structural data available from X-ray diffraction. Finally for an estimation of a relative role of the various contributions to observable NQR frequency shifts, one can resort to theoretical calculations. Intermolecular interactions

Another example arises in the situation where a quadrupole halogen atom makes a symmetrical bridge between two metal atoms, which is often the case in polymeric metal halides of composition MX n (n = 3–5). Dimers of metal halides, e.g. AlCl3, GaBr3, TaCl5, SbCl5 or [Al2Br7]−, have thus been attractive for NQR investigators, because of the differences between the resonance frequencies of bridging (X b) and terminal (X t) halogen atoms. The dimers have structures with either two bridging M–X b–M bonds of about equal length, as in GaBr3, or two bonds of significantly different length, as in complexes of oxygen donor ligands with mercuric halides, or one bridging bond as, e.g. in the aluminium heptabromide anion [Al2Br7]−. Figure 1 presents the structure of one type of symmetric bridging dimeric halide. The spatial structure of the MX 3-type dimer is shown in Figure 1A whereas Figure 1B presents the same structure projected onto the plane of the bridging bonds. The halogen atoms involved in the bridges are, together with metal atoms (M), in one plane which is perpendicular to the plane of the other terminal halogen atoms (X t) and the metal atoms. Analysis of the Zeeman splitting of the NQR spectra of halides of non-transition metals from group IIIA of the periodic table permits a determination of the directions of the main axes of the EFG tensor on the bridging halogen atoms. These directions are marked in Figure 1B. The axis of the greatest gradient of the electric field on the bridging halogen atom is perpendicular to the plane that contains the metal atoms and the bridging halogen atoms. The same axis, but on the terminal halogen atoms, lies along the metal–halogen bond. The orientation of the main

Figure 1 (A) Spatial structure of MeX3 type halides with two bridging bonds. (B) The same structure projected onto the plane of bridging bonds. The directions of the main axes of the EFG tensor on the bridging halogen atoms are marked, in the case when [<109°28′.

axes of the EFG tensor in the case of other types of bridging dimeric halides is similar. We have indicated that the NQR frequencies of the bridging halogen atoms in non-transition metal compounds are lower than those of the terminal atoms and that this relationship is reversed in the case of transition metal compounds. Table 5 includes the NQR frequencies of the bridging and terminal halogen atoms, the values of the EFG asymmetry parameter for the bridging and terminal halogen atoms, as well as the angles at the bridging atoms (see Figure 1B) for some non-transition and transition metal dimers. X-ray structural and electron diffraction data indicate that, independent of the nature of the central metal atom, the length of the bridge is always greater than the distance between the terminal atoms. In non-transition metal compounds the effective negative charge on the bridging halogen atoms is smaller than that on the terminal atoms. The opposite situation is found in transition metal halides. In these compounds the effective negative charge on the bridging atoms is greater than that on the terminal ones. The explanation for this lies in the nature of the metal valence shells (p–d transfer). In the

NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS 1659

Table 5 NQR spectral parameters of some non-transition and transition metal dimer halogenides

Compound GaCl3 GaI3 AlBr3

Q(Xb) (MHz)

Kb (%)

Q(Xt) (MHz)

Kt (%)

[(°)

8.9

86.0

14.667

47.3

19.084 20.225

3.4

133.687

23.7

173.650

0.9

174.589

2.8

113.790

–

82.0

0

93.5

1.1

94.5

97.945

–

94.5

115.450 AlI3

111.017

18.1

InI3

122.728

29.7

129.327 129.763 173.177 173.633

NbCl5

13.290

58.8

7.330

–

101.3

NbBr5

105.850

58.8

59.500

–

101.3

WBr5

114.580

44.9

81.660

–

98.6

framework of the Townes and Dailey theory this means an effective lowering of the resonance frequency owing to the decrease in the px and py orbital population of the terminal halogens. Figure 2 presents a correlation found by the linear regression method between the values of unpaired pelectron densities for the bridging (U ) and terminal (U ) halogen atoms for a large number of compounds. The existence of these correlations is connected with the influence of the p-electron distribution on NQR frequencies, and the bridging and terminal halogen atoms are connected through the central atom, which acts as a buffer. One can see from Figure 2 that the experimental points can be divided into two groups, corresponding to non-transition and transition metal compounds. This is in good agreement with the difference in NQR frequencies for the bridging and terminal halogen atoms for all the compounds studied. As a consequence, the unpaired electron density differs for the compounds of both groups, although U increases with increasing U for all compounds studied. The observed correlations are approximate, which points to the influence of other factors, such as crystallographic

Table 6

Figure 2 U versus U for halogen atoms in halides of transition (o) and non-transition (•) metal elements.

uncertainty and the position of the molecules in the unit cell of the crystal. Charge distribution in biological molecules

NQR spectroscopy can be also used to provide detailed information on the structure and conformation of biologically active systems. It offers a unique possibility of determining the quadrupole coupling constants and, as a consequence, effective charges, and in this way allows a determination of the electronic structure of the molecule. NQR spectroscopy appears to offer a powerful tool for the investigation of various chemical effects in the solid phase of many nitrogencontaining compounds. Analysis of the quadrupole coupling constants of nitrogen atoms allows an estimation of the electron density distribution on the nitrogen nuclei and enables the analysis of charge distribution in chemical bonds involving nitrogen. For example, the effect of substitution at the 1-H position of the 2-nitro-5-methylimidazole [2] ring can be analysed (Table 6), Hitherto, different imidazole derivatives have been studied by 14N NQR, by the continuous wave method, by 13C NMR and 35Cl NQR (pulse methods). However, the problem of the effect of a substituent at the 1-H position on the

Chemical names and substituents of the compounds studied

No.

Compound

R1

R2

R3

R4

[1]

Imidazole

H

H

H

H

[2]

2-Nitro–5-methylimidazole

H

NO2

H

CH3

[3]

1-(2-Hydroxyethyl)-2-nitro-5methylimidazole (metronidazole)

CH2CH2OH

NO2

H

CH3

[4]

1-(2-Carboxymethyl)ethyl-2-nitro-5-[2-(P-ethoxy-phenyl)ethenyl] imidazole

CH2CH2OCOCH3

NO2

H

CH CHPhCH3O

1660 NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS

Table 7 Quadrupole coupling constants and asymmetry parameters for imidazole derivatives

Nitrogen Nucleus N

e2Qqzzh1 No. T (K) (MHz) [1]

[2]

[3]

[4]

NO2

NR K

e2Qqzzh1 (MHz)

K

e2Qqzzh1 (MHz)

K

291 3.222

0.119 1.391

0.930

–

–

77 3.253

0.135 1.418

0.997

–

–

296 3.243

0.250 1.569

0.821 1.225

0.356

193 3.249

0.249 1.546

0.878 1.244

0.360

296 3.299

0.150 2.467

0.317 0.936

0.381

193 3.320

0.156 2.479

0.320 0.950

0.381

296 3.755

0.038 2.566

0.238 0.921

0.239

193 3.779

0.039 2.565

0.238 0.931

0.230

electron distribution in 2-nitro-5-methylimidazole [2] derivatives has not been considered. The results of NMR-NQR double resonance studies on a series of imidazoles (Table 6) are collected in Table 7.

stand for the lone pair population of an N atom, while S is the S-electron density. A comparison of the data collected in Table 8 shows that in the case of a 3substituted nitrogen atom, the lone pair electron density and NR bond population increase with increasing length of a substituent at 1-H position in the imidazole ring, both at 193 and 269K. The substitution of an NO2 group at the 2-position and a methyl group at the 5-position of the imidazole ring leads to an insignificant redistribution of electron density (of the order of 2%) relative to that in pure imidazole. Only the introduction of a substituent at the 1-H position of the ring causes considerable changes. Similarly for a 2-substituted nitrogen, the bond population changes in a characteristic way – the S-electron density and population of the N–C bond decrease with elongation of the substituent at 1-H position of the imidazole ring (Table 8). Interestingly, a qualitatively similar but much more effective phenomenon is the electron redistribution on the nitrogen atoms of the NO2 groups. Therefore it can be concluded that, in the case of 2-nitro-5-methyl derivatives of imidazole, with increasing length of the chain of the aliphatic substituent at the 1-H position of the imidazole ring, the electron density of the S-orbital and V-bond of the 2substituted atom, as well as the S-orbital and V-bond of the NO2 group towards the 3-substituted nitrogen, undergoes redistribution. Of essential importance is the character of substituents, i.e. CH2CH2OH and CH2CH2OCOCH3 groups are electron density acceptors, while for 1H-imidazole and 1-acetylimidazole, the opposite tendency was observed, i.e. the S-electron density and the population of the V-bond increased. The results also indicated that the effect of temperature on the bond populations in the imidazole derivatives studied is negligible. The 4-N-derivatives of cytosine [5], R-substituent R = H, CH2Ph, CH2CH2Sh, CH2CH2Ph and naphthaTable 8 Population of the NH–, N ,NO2 bonds for imidazole and its derivatives NH

As follows from the data in Table 7, the introduction of NO2 and CH3 at positions 2 and 5 of the imidazole ring, respectively, leads to an increase in nuclear quadrupole coupling constants on both nitrogens of the ring and a decrease in the value of this parameter on the nitrogen from the NO2 group. Results of a bond population analysis carried out according to the Townes–Dailey method for imidazole and its derivatives are displayed in Table 8. According to the assumed notation nNC and nNR stand for the population of N–C and N–R bonds, na

No. na [1]

[2]

NO2

N

nNC

nNR

p

nNC

p

nNC

T(K)

1.330 1.120 1.330 1.640 1.260

–

–

77

1.340 1.140 1.330 1.640 1.270

–

–

293

1.350 1.129 1.330 1.657 1.556 1.072 1.182 193 1.361 1.139 1.330 1.652 1.556 1.070 1.178 296

[3]

1.672 1.250 1.368 1.449 1.390 1.042 1.128 193 1.669 1.250 1.366 1.452 1.394 1.041 1.126 296

[4]

1.648 1.250 1.340 1.430 1.384 1.045 1.117 193 1.648 1.250 1.340 1.430 1.384 1.044 1.116 296

NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS 1661

lene, have aroused significant interest mainly because of their biological significance. 4-N-methyl and 4-Nacetylcytosine have been found in nuclei acids as rare bases. The results of the analysis of bond populations for cytosine and its derivatives performed according to the Townes–Dailey theory are described below. On the NH nitrogen, the changes in electron density distribution induced by substitution are insignificant. The symmetry of charge distribution at the NH nitrogen in 4-N-thioethylcytosine is the lowest while in 4N-naphthalenecytosine it is the highest. On the other hand, the greatest value of the z-component of the EFG tensor was detected in the vicinity of the NH in pure cytosine while the smallest was in naphthalenecytosine, which implies that the electrons are drawn away from NH by the system of bonds depending on the substituent. Thus, it can be concluded that the introduction of a substituent at the amine group of cytosine at 4-N does not cause any changes when the substituent contains a chain such as CH2CH2 which separates the aromatic system from the cytosine ring. However, when the aromatic system is separated only by one CH2 group, or is not separated at all, there occurs a strong inductive effect which is responsible for drawing S-electron density from the NH nitrogen. For a 2-substituted nitrogen the symmetry of charge density at –N= is 50% lower. This is if one compares that of 4-N-phenylmethylcytosine with that of 4-Nthioethylcytosine, and about 20% lower than that of 4-N-naphthalenecytosine. On the other hand, the quadrupole coupling constant and thus the z-component of the EFG tensor at the –N= nitrogen, is much higher in the derivatives whose substituents are not separated by the CH2CH2 chain. The difference between the population of the NC bond for pure and substituted cytosine is small −0.0001 for 4-Nthioethylcytosine but as great as 0.132 for 4-N-phenylmethylcytosine, which is still much less than the difference between the populations of V and S bonds. Such a situation indicates that the changes in V-bond population are dominant. In 4-N-naphthalenecytosine the change in S-electron density is dominant. This implies that –N= in phenylmethylcytosine and naphthalenecytosine plays the role of a buffer. In 4N-phenylmethylcytosine and 4-N-naphthalenecytosine the electron density of a free electron pair at the nitrogen NHR is significantly delocalized. The electron density on the NH bond in 4-N-naphthalenecytosine, 4-N-phenylmethylcytosine and 4-Nthioethylocytosine relative to that in pure cytosine changes by 0.053, 0.038 and 0.005, respectively, while for 4-N-phenylethylcytosine there is no change. Thus, the amine group which acts as S-electron acceptor in the majority of molecular systems, becomes the electron donor in phenylcytosine and naph-

thalenecytosine. The aromatic rings which usually compensate changes in electron density in cytosine act as electron acceptors. When the aromatic substituents are separated by the CH2CH2 chain, the density redistribution is reduced, which is in agreement with the tendency observed for chlorobenzenes. Investigations of cytosine and its derivatives have shown that the cytosine derivatives with an aliphatic substituent do not show an anticancer activity, but those with an aromatic substituent do. However, the mechanism of this activity has not been explained. The results suggest that in the search for anticancer drugs from the group of 4-N-cytosine derivatives, the choice should be those in which the aromatic substituents are not separated by a CH2CH2 chain, whose presence induces a significant redistribution of S-electron density. The 14N NQR frequencies recorded for cytosine derivatives were practically the same at ambient and liquid nitrogen temperatures, which means that no essential redistribution of electron density occurs in this temperature range. The 14N nucleus, because of its widespread occurrence in all types of systems (especially biologically active systems), is of particular interest in studying electron density distribution, molecular reorientations and intermolecular time-dependent interactions. It seems that such studies will acquire more and more importance in the future and will occur more frequently, especially with the availability of double resonance spectrometers and new data processing techniques such as the maximum entropy method. The examples discussed do not, of course, exhaust the potential of NQR as a tool for structure and chemical bonding. These are only simple illustrations of the applied aspects of NQR spectroscopy. Application of NQR to the detection of explosives, contraband etc.

The increasing use of plastic explosive and drugs has found experts and research facilities scrambling for new detection methods. One of these ‘new’ methods is NQR. In the past five years a number of researchers around the world have independently begun to reconsider NQR as a possible solution for the detection of plastic explosives and started developing it specifically for bomb and narcotics detection. The noninvasive nature of NQR (closely connected with the absence of magnets) gives it some advantages over other methods. Pulsed-RF NQR produces single, or nearly single, peak signals at specific frequencies that depend on the specific bond environment surrounding an element in a given compound, usually a crystalline solid. Because the resonance frequency is almost unique to each compound, NQR exhibits great specificity for

1662 NUCLEAR QUADRUPOLE RESONANCE, APPLICATIONS

analytes such as explosives and narcotics. The most useful elements to monitor by NQR are 14N, 35Cl and 37Cl. Most high explosives contain 30–40% N and a large number of drugs such as cocaine and heroin are prepared as chloride salts. Most pure explosive such as RDX, HMX, TNT, C-4 are crystalline or semicrystalline compounds embedded in a polymer matrix, rather than pure polymeric compounds, so that they are immobilized and relatively ordered and as such give good NQR. Of course liquids and polymers are too disordered to give an NQR signal, although some monomers have shown detectable resonances. NQR can be also used to differentiate between explosives, narcotics and benign nitrogen-containing compounds such as polyurethane foam or nylon. False alarms from these compounds are not the problem for NQR whereas, they might be for neutron activation or other generic nitrogen-detecting methods. It is well known that commercial and military explosives are physical mixtures of pure explosive compounds with some additive plasticizer or binder. Because NQR is so compound-specific, physical additives do not interfere with the signal for a target compound so NQR can be used to identify explosives that are not in a pure form. Moreover, it does not matter what form the explosive or drug is in – whether it be tin sheets or small pellets. NQR may eventually be used to detect bombs or narcotics with spatial resolution, in the same way as X-ray metal detectors. NQR is inherently less flexible than NMR but when it works it is extremely attractive because of its specificity. NQR can work with slurries, aggregates and possibly even emulsions, as long as the molecular dynamics are slower than the NQR method time scale (the MHz range). The ongoing use of NQR as analytical tool for measuring solid-state phase transitions and order–disorder in materials may also be of interest.

Summary NQR is not as extensively useful at present as NMR spectroscopy. The best results on light nuclei, such for example as those on 27Al nuclear coupling constants in mineral samples, have been made using NMR. Here, the changes in the NMR spectra were considered as a function of the orientation of a single crystal in external magnetic field. NQR could, however, directly measure the same nuclear coupling constant data using neither single crystals nor an external magnetic field. NQR measurements possess high spectral resolution, precision, specificity and speed of measurements. The reason for the relatively

limited practical application of NQR seems to lie in the lack of sufficiently sophisticated equipment. NQR applications can be divided into four groups: 1. Studies of the electron density distribution in a molecule – changes in orbital populations under substitution, and complexation. 2. Studies of molecular motions – reorientations, rotations, hindered rotations. 3. Studies of phase transitions. 4. Studies of impurities and mixed crystals.

List of symbols e2Qqzzh–1 = quadrupole coupling constant; e2Qqph–1 = quadrupole coupling constant; I = nuclear spin quantum number; IB = ionic character of a bond; qp = the electric field gradient produced by one unbalanced p electron; qzz = electric field gradient (EFG) at a quadrupole nucleus; U , U = unpaired pelectron density of a bridging and terminal, halogen, respectively; K = asymmetry parameter; J = Sternheimer antishielding factor. See also: Mossbauer Spectrometers; Mossbauer Spectroscopy, Applications; Mossbauer Spectroscopy, Theory; NQR, Theory; Nuclear Quadrupole Resonance, Instrumentation.

Further reading Buslaev JA, Kolditz L and Kravcenko EA (1987) Nuclear Quadrupole Resonance in Inorganic Chemistry. Berlin: VEB Deutsche Verlag der Wissenschaften, 237 pp. Das TP and Hahn EI (1958) Nuclear Quadrupole Resonance Spectroscopy. Solid State Physics, suppl. I. New York: Academic Press, 223 pp. Gretschischkin WS (1973) Yadernye Kwadrupolnye Vzaimodejstviya v Tverdych Telach. Nauka: Moskva, 264 pp. Lucken EAC (1969) Nuclear Qudrupole Coupling Constants. London: Academic Press, 360 pp. Safin IA and Osokin D (1977) Yadernyj Kvadrupolnyj Rezonans v Soedinieniach Azota. Moskva: Nauka, 255 pp. Semin GK, Babushkina TA and Yakobson GG (1975) NQR Group of INEOS AN SSSR, Nuclear Quadrupole Resonance in Chemistry, (English edition). London: Wiley, 334 pp. Smith JA (1974–1983) Advances in Nuclear Quadrupole Resonance, Vol. 1–5. London: Heiden & Sons. Townes CH and Dailey BP (1949) Determination of electronic structure of molecules from nuclear quadrupole effects. Journal of Chemical Physics 17: 782–796. Townes CH and Dailey BP (1955) The ionic character of diatomic molecules. Journal of Chemical Physics 23: 118–123.