Wayne L. Smith Colby College Waterville, Maine 04901
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The introductory pres~mtntionsof nuclear magnetic resonanct. can be quite frustratin:: for an inorganic chemist. The rmphasis is largely on proton nmr and organic compounds with little menticm of thc urilitvand s~:~,l,e~)ftl~ispurr~erii~l tool in inoreanir "~ resmrch. For eramde. "B. '"I.', and "P nrnr have proved invaluable in product'idktification and structure elucidation of compounds containing these elements. Although examples of the utility of non-proton nmr can be found (1-8), they often appear in specialized monographs which are not widely available. The present article is intended as an introduction to the general ~rinciplesof "B nmr and a convenient compilation ofseveral specific examples. It should prove useful for senior level inorganic courses as well as courses in instrumental analysis and spectroscopy. There are several reasons for the lag in appreciation of the utility of l l B nmr. Among them are: (1) Boron chemicals do not have the far reaching industrial applications that carbon compounds enjoy (9); (2) In general only large university and ~overnmentlabs are equipped to obtain "B spectra; (3) cornoared to nroton nmr. the resolution of llB nmr is rather poor:~i,13md'ing in the higher horon hydridt.~isnot as rendilv exrhinrd ;I.; that in hvdntiarhons: (61 It ismuch morediitin~lt tokstahlish "rules oE thumb" concerning chemical shifts in 'IB nmr; (6) In I1B nrnr individual peaks are so hroad that overlapping of peaks is the rule rather than the exception; (7) "B-'H coupling constants are frequently larger than the relative chemical shifts; (8) There is no single material that is as ideal as a chemical shift standard as tetramethylsilane is for proton spectra. Some of these difficulties are being overcome. For example, the resolution problem is being solved through use of higher field strength spectrometers and Fourier transform techninues. And as more and more snectra become available it is possible to recognize a few patterns and correlations. Most mvestieators now reoort chemical shifts relative to trifluoroborah etherate (E&O.BF~),and this paper will follow that practice. Despite the difficulties, the hroad range of structures possible with horon compounds, and often the relative simplicity of the spectra, makes I L Bnmr a valuable tool for confirming or establishing geometry of these compounds. ~~
Principles of "B NMR The general principles of nmr spectroscopy have been adequately discussed elsewhere (10) and only a hare outline will he nresented here. An earlier article in this Journal (11) provides an excellent introduction to the differences between ' H a n d "B nmr There are two stable isotopes of horon, 1°B and "B, with natural ahundances of 19.6 and 81.4%, respectively. Both isotopes possess magnetic moments and thus both exhibit characteristic nmr absorptions. The l l B isotope has proven more useful for nmr experiments for several reasons: (1)"B signals are more intense due to its greater natural abundance. (2) "B has a larger nuclear magnetic moment than l0B (2.688 and 1.80, respectivelv, in units of the nuclear magneton). In general. the nmr signal-to-noise mtio increases with increasing nuclear moment. I:$) The spin I in units of hl2a uf "R is :1 2 whilr that of "'13 is 2. Thus the IoB isotone has a larner ~~~~- n~lclenr quadrupole moment and its nmr spectra are not as sharp as those of "B. In a uniform magnetic field, H, a given nucleus can assume ~
~
.~ ~
Boron-I I NMR +
any one of 21 1 possible orientations relative to the applied field. The nucleus will nreress about the anplied field at an .. angular velocity yH,where y is the magnetogyric ratio (a characteristic constant for each nucleus). Each orientation corresponds to an energy E = -yhH- M ~
~
~~
2r
where h is Planck's constant and M has one of the following values: I, I - 1,I - 2,. . . -1. Thus the energy level diagram for a "B nucleus in terms of M is as follows
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Thus a proton coupled to a llB nucleus "sees" four different fields, all equally probable. All transitions are degenerate, and the 'H nmr spectrum of a proton coupled to a l l B nucleus will he a quartet with all four peaks of equal intensity. Similarly we would expect one "B nucleus coupled to another "B nuto exhibit a air of auartets. assumine that the two nuclei cleus -~~ were in sufficiently different chemical envimnments so that overlapping of peaks did not occur. This is equivalenr to saving that the couplinl: constant, J I ~ , . ~ must ~ , , be murh less than the chtxnical shift. 6. Fortunacelv. such hor~,n-horoncourhng is not observed &e to the l a r k quadrupolar momend and resultant broadened resonance signal in the "B nrnr spectrum. Most "B nmr spectra are of horanes and thus involve coupling of the horon nucleus with a proton. Since the 'H nucleus has a spin of 112, the "B nucleus can he in either of two net fields. The 'H spin may he parallel or antiparallel to the external field. Thus the llB nmr absorption will he a symmetric doublet. The Larmor frequency for "B appears a t lower energies than that for 'H by a ratio of 113.117. Thus on a 60 MHz proton spectrometer, I1B js observed a t 19.2 MHz; on a 100 MHz proton spectrometer, "B is observed a t 32.09 MHz; and on a 200 MHz proton spectrometer, "B is observed a t 70.59 MHz. Most hieh resolution soectra obtained now are a t 32.09 or 70.59 M ~ Zusually , wi& Fourier transform spectrometers ~~
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Interpretation Of Representative Spec'ra
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The simplest system in which "B andlH nucleiare coupled is the borohydride anion, BHn-. The tetrahedral symmetry of BHt- has been inferred from neutron diffraction data on crystalline KBH1(12). Hence all four protons are magnetically equivalent. The "B nmr spectrum is thus a symmetrical quintet (2nZ + 1 = 5 for coupling with four equivalent protons), with a relative intensity pattern of 1:4:6:4:1 as shown in Figure 1.As in proton nmr, multiplets in"B spectra have lines with intensities corresponding to the coefficients in the binomial expansion. 64H10
Tetraborane(lO),B4Hlo,has a "butterfly2'shapewith four bridge hydrogens and six trrnainnl hydn,:mr 'l'hir structure was prrwourly wt;rhiiahed ly X-ray diffraet~onstudies $141and is nmfirmrd by the "13 n m r s~ectrum11.51 sketched in Fiaurr 2. The Lou lidd rrvlet arises from coupling of horons 2 and 4 4 t h terminal protons and~the Volume 54, Number 8. August 1977 / 469
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Figure 1. 6 MHz "B nmr spectrum of NaBH4 in H20. Ref. (3) and ( 13).
12.4)
66.9 640 11.31 128 151 Figure 2. 12.6 MHz "B nmr spectrm of neat B4Hlo.Ref. (*and (15).
high field doublet arises from coupling of borons 1 and 3 with their single terminal protons. The fine structure on each boron resonance ran be rationalized as a poorly resolved triplet structure due to coupling with bridge protons. The claim 115) that 'OB-"B coupling occurs was shnwn to be inenrrect by an examination of the "B nmr spectrum ,rf'RID1ll116). In the deuterated species both signals collapse to singlets. Recause the bridge protonsare closer to the central borons 1and :I 11.33 A) than to bornns 2 a n d 4 (1.43A) 1171, it is reasonsble t o e r pect better resolved triplets for the BH? resonances. This hyperfine structure due to "B-'H coupling is not observed in the other boron hydrides. B5Hs Pentaboranel9). BsHn, has a tetragonal pyramidal structure. Each boron has one terminal hydrogen atom, and there are four bridge hydrngens bonded to the four basal borons. This structure has been established in the vapor state by electron diffraction (18) and micnwave spectroscopy 119) and also in the crystalline state by an X-ray diffraction studv (20).The "B nmr soectrum consists ofa high field
b Figure 4 ( a )32.1 MHz "B nmr spechum of l-CIBsHa.Ref. (22).( b ) 32.1 MHz "0 nmr spectrum of 2X1BsH8. Ref. (23).
the four basal borons coupled to terminal protons; coupling with the bridee orotons is not observed. One might nnticiparr that the bawl horons would he more highly shwld~rlnnd r h u ~ilppear upfield. This is n
