Inorg. Chem. 1983, 22, 1849-1853
1849
Contribution from the Department of Chemistry, The Open University, Milton Keynes, MK7 6AA England, and Rocketdyne, A Division of Rockwell International, Canoga Park, California 91 304
Nitrogen-14 and Nitrogen-15 NMR Spectroscopy of Fluoronitrogen Cations: FIuoro Effects
T
and
B
JOAN MASON* and KARL 0. CHRISTE Received September 30, 1982
High-resolution spectra have been obtained for (anhydrous HF) solutions of NOF2+,NF4+,NH4+,NO2+,and NO+ in I4N resonance and for NH3F+(in CF3S03H)in I5N resonance. Broader 14Nlines were obtained for N2F+(although the one-bond NF coupling was resolved), N2F3+, and NH3F+. For F2N=NF+ and NH3F+the reduced electronic symmetry promotes quadrupolar broadening of the 14Nline; for N2F+and F2N=NF+ exchange processes may contribute also. The nitrogen lines in the linear or planar species N=NF+, NOF2+,and F2N=NF+ show ?rfluoro effects, being shifted upfield relative to those in corresponding species with hydrogen, alkyl, or aryl groups instead of fluorine, despite the reduction in electron density on nitrogen. The higher shielding is related to increase in energy of nN ?r* and u a* paramagnetic circulations and so corresponds to perfluoro effects which are well-known in electronic and photoelectron spectroscopy. In planar systems, fluorination stabilizes u relative to ?r orbitals, since interaction with the filled F, orbitals counteracts the inductive stabilization of the a orbitals. In the nonplanar species, however, the nitrogen line moves strongly downfield with fluorination, as from NH4+to NH3F+to NF4+. These shifts are described as ufluoro effects and are explained, at least in part, by the decrease in electron density on nitrogen. The higher shielding of nitrogen in NH4+in anhydrous HF relative to that in aqueous solutions can be attributed to N-H-F hydrogen bonding.
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Introduction Because of the extreme position of fluorine in the periodic table, effects that occur to a lesser degree with other substituents can be "tested" in fluorine chemistry; indeed some are then so marked as to be called "(per)fluoro effects". An example is the perfluoro effect in planar systems which is used to distinguish u from a orbitals in photoelectron spectroscopy and to characterize n a * (or u a*) excitations in electronic It results from the marked stabilization of the a-orbital manifold relative to the a when hydrogens or alkyl groups are replaced by fluorine. Although the (-1) inductive effect of fluorine stabilizes the u and the a orbitals, the effect on the a orbitals is offset by the repulsion of the fluorine nonbonding electrons (+Ir). Corresponding effects can be discerned in 13C and lSN N M R shift^.^ In the azabenzenes,& for example, nN a * bands are strongly blue shifted and nitrogen resonances shifted upfield, since (planar) nitrogen carrying a lone pair is deshielded by nN a * electronic circulations in the magnetic field, and an increase in the excitation energy U ( n N a*) acts to reduce the circulation and the deshielding. Such "perfluoro" effects are usefully (following Liebmad) termed a fluoro effects, as they are evident also on partial fluorination, with some additivity. The term " u fluoro effects" can then be applied to nonplanar systems (in which dramatic downfield shifts may be observed for atoms directly bonded to fluorine) and also to contributory influences of fluorine attached to a resonant atom in a a-bonded system. These effects reflect changes in electron density and orbital coefficients as well as in excitation energies, as discussed below. We now report a nitrogen N M R spectroscopic study of the cations NF4+,' NH3F+,8 FzN=NF+,9 NOFZ+,l0F'N=N+ l1,l2 NH4+, NO+, and NOz+, in anhydrous HF (or CF3S03H) solution, to throw light on the effects of fluorination in these ions.
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field gradient) allows the nuclear electric quadrupole and therefore the nuclear spin to relax sufficiently s10wly.~ Thus high-resolution I4N N M R spectroscopy should in principle be possible for the NF4+, FN=N+, and FzN=X+ ions, but greater quadrupolar broadening is expected for the FzN=NF+ nitrogen, which carries a lone pair of electrons, although lJNF and 2JNF were resolved in I9F(l4N1double resonance studies of cis- and trans-FN=NF.I3 (l4NI4Ncoupling constants are expected to be small, 5 Hz or less, since 'JISNISN is about 6 Hz for the dinitrogen ligand M - N E N ' ~ ~ or 10 Hz for the hydrazido(2-) ligand M=N-NH2.14b) The low viscosity of fluoro compounds and liquid HF as solvent is advantageous for 14N work since the quadrupolar relaxation rate is proportional to the molecular reorientation time and therefore to the viscosity. Pure liquid H F has a viscosity of 0.26 CPat 0 OC and 0.45 CPat -45 "C (cf. 1 CP at 20 "C for water). Unfortunately this solvent is (understably) unpopular with operators of widebore spectrometers for (1) Bralsford, R.; Harris, P. V.; Price, W. C. Proc. R. SOC.London, Ser. A 1960. 258. 459. Potts. A. W.: Lemoka. H. J.: Streets. D. G.: Price. W. C. Philos. Trans. R. SOC.London, Ser. A 1970, No. 268, 59. (2) (a) Robin, M. B. "Higher Excited States of Polyatomic Molecules"; Academic Press: New York, 1974; Vol. 1, Chapter I; Vol. 2, Chapter VIA. (b) Zbid., Vol. 1, Chapter IIID. (c) Zbid., Vol. 1, Chapter IIIB. (3) Brundle, C. R.; Robin, M. B.; Kuebler, N. A.; Basch, H. J . Am. Chem. SOC.1972, 94, 1451. (4) (a) lac:Mason, J. J. Chem. SOC.,Faraday Trans. 2 1979, 75,607. (b)
(5) (6) (7)
"N: Kanjia, D.M.; Mason, J.; Stenhouse, I. A.; Banks, R. E.; Venayak, N. D.J . Chem. SOC.,Perkin Trans. 2 1981,975. Mason, J. J. Chem. SOC., Faraday Trans. 2 1982,79, 1539. (d) lsN, nlP, "0,77Se:Furin, G. G.; Renukhin, A. I.; Fedotov, M. A,; Yakobson, G. G. J . Ffuorine Chem. 1983, 22, 231 and references therein. Mason, J. Chem. Rev. 1981, 82, 205. Liebman, J. F.; Politzer, P.; Rosen, D.C. "Applications of Atomic and Molecular Electrostatic Potentials to Chemistry"; Politzer, P., Truhlar, D.M., Eds.; Plenum Press: New York, 1981. (a) Christe, K.0.;Schack, C. J.; Wilson, R. D.Znorg. Chem. 1976, 15, 1275. (b) Christe, K. 0.; Guertin, J. P.; Pavlath, A. E.; Sawcdny, W. Zbid. 1967,6, 533. (c) Tolberg, W. E.; Rewick, R. T.; Stringham, R. S.; Hill, M. E. Zbid. 1967, 6, 1156. Grakauskas, V.; Remanick, A. H.; Baum, K. J . Am. Chem. SOC.1968,
14N vs. lSN NMR Spectroscopy Nitrogen N M R spectroscopy in high resolution normally requires the ISN nucleus, but the low abundance (0.365%) has severely restricted its application to fluoronitrogen chemistry. Sharp lines can, however, be obtained for the abundant but quadrupolar 14N nucleus in mobile solutions of NH4+, CH3N e , or NO), since the high local symmetry (small electric
90, 3839. (9) Christe, K. 0.;Schack, C. J. Inorg. Chem. 1978, 17, 2749. (10) (a) Christe, K. 0.;Hon, J. F.; Pilipovich, P. Inorg. Chem. 1973, 12, 84. (b) Christe, K. 0.; Maya, W. Zbid. 1969, 8, 1253. (11) Moy, D.; Young, A. R. J . Am. Chem. SOC.1965,87, 1889. (12) Christe, K. 0.; Wilson, R. D.;Sawcdny, W. J . Mol. Struct. 1971, 8, 245. (13) Noggle, J. H.; Baldeschwieler, J. D.;Colburn, C. B. J . Chem. Phys. 1962, 37, 182. The N2F4and NF, measurements are quoted from:
*To whom correspondence should be addressed at The Open University.
(14) (a) Chatt, J.; Fakley, M. E.; Richards, R. L.;Mason, J.; Stenhouse, I. A. J . Chem. Res., Synop. 1979, 44. (b) Chatt, J.; Fakley, M. E.; Richards, R. L.; Mason, J.; Stenhouse, I. A. Zbid. 1979, 322.
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Randall, E. W.; Baldeschwieler, J. D., unpublished results.
0020-1669/83/1322-1849$01.50/0
0 1983 American Chemical Society
1850 Inorganic Chemistry, Vol. 22, No. 13, 1983
Mason and Christe
Table I. n-Bonded Fluoronitrogen and Related Compoundsi compd
solvent
FNrN'AsF,' 1 2
N-1
T/"Ca
HF
6(N)b -191.2
' J I ~ N F* / J~~NF/ HzC HzC WI,,/Hzd 339 (14) [3281
nr
120 (12)
nr
400 205 (14) 600
-166.1
nr
-50 PhN=N+BF,1 2 NOF,'AsF,-
-156.4 -63.4 -99.0
N-1 N-2
HF
NO, F
neatliq
N-1
F,N=NF+AsF,1 2
-110
HF
-87.6 -75.9 26.0
254 (3) [2501 109(5) [ 112.51 nr
. ..
1 8 (3)
195 (15)
CCI,F CCI,F neat liq
CF,N(O)=NF 1 2
N-1 N-2
-80 -78 -123 -58
4 66 104 110
k145 +I36
...
18 (6) 10
...
24 870 (50) 2200 280 (20) 950
nr nr
-50 cis-FN=NF trans-FN=NF NOF
ref (14915N)g
19 (ISN)
-50 N-2
ref (I9F)I 11
-50 N-2
Tq/mse
737 313 nr
13 13 21 24 h
245
a Other than ambient temperature. I4N shift relative t o neat liquid CD,NO,, with low field positive. The new measurements were made a t 28.9 MHz (400 MHz for protons) except for NOF,+AsF,-, which was measured at 4.33 MHz (60 MHz for protons). The reference for shifts measured at 4.33 MHz is 5 M NH4N0, in 2 M HNO,, for which NH4+(aq)has 6 -360.0 relative t o neat liquid CH,NO,. J 1 5 ~ / J 1 = 4 ~ -1.403. nr means "not resolved". The spin-spin coupling unresolved in 14N resonance has not been resolved in I9F resonance. Coupling constants shown in brackets were measured in "F resonance. Line width at half-height. e Quadrupolar relaxation time, given by T , = l/nWl,, when the line is not broadened significantly by unresolved coupling or exchange. Reference to 19F measurement of JNF. Reference t o nitrogen NMR measurement. Frazer, J. W.; Holder, B. E.; Worden, E. F. J. Inorg. Nucl. Chem. 1 9 6 2 , 2 4 , 4 5 . ] Uncertainties are given in parentheses in units of the last digits.
Table 11. Nitrogen Oxo Ions compd
solvent
NO,+AsF,NO,+BF,- (FSO,-) NO,+HSO,-O,N=NO-Na, '+ NO+AsF,NO+HSO,NO+BF,' (PF;) NH,+NO; (5 M ) Na'NO;
T/"P
6(Nib
W,,,/Hz
TU/ms
-70 -60
-136.3 -131.5 -132 (1) -43.4 -27.9
5 (1)
65 (30)
-7.5 -60
-5 (10) -3.2 -4.5 229
ref 34 (I")
29
11
C
16 95 (14) broad
3 (0.3) 21 34 (15N)
Chew, K. F. unpublished results. Quoted by: Logan, N. In "Nitrogen NMR"; See footnote b of Table I. a See footnote a o f Table I. Witanowski, M., Webb, G. A., Eds.; Plenum Press: London, 1973; Chapter 6.
the study of I5N in natural abundance when the sample volume is 12 cm3 or more, particularly if the solute is under pressure. Triflic acid (CF3S03H)is more acceptable, and we used this for the I5N spectrum of NH3F+. Results and Discussion As recorded in Tables 1-111, the 14N lines are very sharp for NF4+ and NH4+, quite sharp for NOF2+and NO2+,but rather broad for NO+, and broader still for N=NF+, FN= NF2+, and NH3F+. For solutions of similar viscosity, the quadrupolar broadening should perhaps increase as NF4+ < FzNO+ < F 2 N = N F IN = m < N E N P
