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Nuclear magnetic shielding anisotropies from proton multiple pulse spectra
:CHEhlICAL
PIIYSICS
2 (1973) 7684..@
NORTH-HOLLAND
PUBLISHING
COMPANY
‘,
NUCLEAR MAGNETIC SHIELDING ANlSOTROPrES FROM PROTON MULTIPLE PULSE SPECTRA. U. HAEBERLEN and U. KOHlSCHtiTTER Mar-Ptanck-lnstihrt
JLtirmedtiinische
Forschrmng, A breilungftir Received
molekulore Physik. 69 Heidelbrrg.
Germany
17 May 1973
Prolon multiple pulr speclra of powder samples of anhydrous osalic acid, oxalic acid dihydrate. malonic-. succinic-, phthalic-. maleic- and fumaric acid. succinic-. phthalic- and maleic acid anhydride, and fcrroccne have been recorded 31 90 hltlz. The spccln arc analysed in terms of the nuclear magnetic shielding ankorropy do =a~ - ul. No evidence for non-axhlly symmetric shielding has been obtained. The shielding anisouopics seem to reflect PI least qualitatively the immcdiale electronic environment of the protons under study. The anisotropies obtained for carboxylic, aliphalic. olefinic and aromatic protons arc = +lS, J +lO, = * 0 and 3 -5 ppm with substantial varialions among the various compounds studied.
1. Introduction Soon after the discovery of the phenomenon of nuclear magnetic shielding or “chemical shift” by Proctor in 1950 [l] a rapidly increasing flow of chemical shift determinations.
mostly
of protons,
set in. Apart
from a very few exceptions [2], however, only the isolropic part or the trace of the nuclear magnetic shielding tensor was accessible to measurement. With the advent of new techniques in recent years - oriep tation of molecules in nematic solvents [3), multiple pulse sequences [4], double resonance techniques [5] - and the availability of homogeneous high magnetic fields [6] there is now a similarly increasing flow of data about the anisoiropy of ‘he nuclear magnetic shielding. However, the experimental informa:ion now available is mostly about 19F and 13C, to a lesser extent about 31P, while 1H is definitely lagging behind. In 1969,the MlT group ran a few proton multiple pulse spectra [7] but to our knowledge only one serious NMR multiple pulse study has been undertaken since then on protons [8]. There are a few proton NMR investigations of molecules oriented in neniatic solvents [3,9], but these suffer from a general shortage of information as discussed in ref. [ 1 I]. This situation is caused by the fact that the isotropik as well as anisotropic nuclear magnetic shielding is usu,ally much smaller for hydrogen than .for all other
elements, whereas the (homogeneous) dipolar.coupling of the nuclei which tends to mask the anisotropy of the shielding is strongest for protons. We have applied multiple pulse methods to a number of powder samples of dicalbonic acids and their hydrides. All rtsults can be represented by axially symmetric shielding tensorsa. Au = u,, - uI is found to be positive and comparative!y large for carboxyl protons (== +I8 ppm), smaller but with a quite large variation for ahphatic protons (+6 ppm to +I0 ppm). below resolution (2 ppm) for olefinic protons, and negative for aromatic protons (z -5 ppm).
2. Experimental 2. I. A pparahrs We used a modified 90 MHz Bruker pulse spectra meter for this work. The power amplifier and the receiver were specially trimmed for the requirements of multiple pulse experiments. The pulse power available is in excess of 2.5 kW. Owing to the electronically regulated high voltage.power supply (Bruker) there is no noticeable droop during long pulse trains. Pulse rise and falltimes are about 100 nsec (10% cf 90%). We usually place a 6 dB attenuator between the transmitter and the (homemade) probe. This helps
II. Haeberlen and U. Kohlschiittcr.Proton shielding
greatly to reduce ringing between the tuned tank circuit of the power amplifier and the probe which itself has a bandwidth of 10 MHz. With the 6 dB attcnuator in the line, 90” pulses are about 700 nsec and the nuclear signal is faithfully detected 1.I3WC after the beginning of the rf pulses. After digitization (Bunker Ramo. Model BRSSO) the data are transferred via the direct access to memory to a general purpose ccmputer (Model 10010, C.I.I., Paris) where they are stored and accumulated, if necessary. After minor manipulations in the 10010, the data are transferrcd to the disc of a (larger) 10020 computer (C.I.I.) of which the 10010 is a satellite. All further data processing including Fourier transformation is done in the 10020 under control of the 10010. Data can be called from the disc to be displayed on a large CRT screen near the NMR spectrometer. The availability of on line Fourier transformation and immediate display of time and frequency data is essential for this type of experiments. In our spectrometer, the timing of the pulse SCquences is derived from the 10 MHz quartz oscillator of a 100 MHz frequency synthesizer (Schomandl). In order to maintain
full coherence between pulse edges ar.d rf, we work at 90.000 MHz. Consequently we must provide for a means to monitor the dc magnetic field, preferably in a precisely controlled manner. An external NMR field lock (Bruker) referenced by a fre-
anisolropik
77
width effects, phase glitches and small phase misalignments [ 131. Throughout this investigation r was chosen to bc 4 WC. With the output power of the transmitter some, what reduced from maximum (reduced Screen voltage), we iint adjust the width of the 90” pulses in the LX. +y channels by observing the NMR response of a liquid sample to a sequence of nominally 90” pulses. We then adjust the --x channel phase 180” off the x channel phase which is fwed. This is done by applying a (pX - T-f’_,-T) n sequence to the liquid sample. The criterion is a vanishing beat in the response. The most critical adjustments
are the phases of the *_v channels which are to be in quadrature to the +x channels. Usually one has to rely on the adjustment of a maximum NMR signal, which varies as cos 0. fl is the phase misalignment. This does not provide for a sharp criterion for 0. In order to obtain a better criterion, which actually has the sensitivity of a sin /3 dependence, we adopted the procedure $E”scribed in the legend of fig. I. After, say, they channel phase has been brought in quadrature to the x channel phase, the --y channel phase is made oppcsite to they channel phase in the same way as described for the ti channels. If now the pulses in all four channels are switched on to give the [ 1.3,21 sequence the observed beat
quency synthesizer, serves for that purpose. It turned out that the main output of the Schomandl synthesizer contains a lot of sidebands which interfere with the NMR signal and show up as a 1 MHz, 100 kHz and 10 kHz ripple on the NMR signal. The baseline is free of this ripple. In order to circumvent this problem, we now use as the basic frequency source of the spectrometer a times 9 frequency multiplier, which is also driven by the buffcred 10 MHz quartz oscillator and which is followed by a sharp bandpass filter. 2.2. Alignment procedure For the earlier part of this work we used the WAHUHA four pulse or [ 1,3,20 cycle in the notation introduced by Mansfield [ 121. The more recent spectra were obtained with the [1,3,2; 1,7,21 [ 1,3,2; 1,7,2]lt sixteen pulse cycle which is self-compensating with respect to rf inhomogeneity, finite pulse
Fig. i. Scheme to adjust quadrature phases in two pulse channels. cg.. the-x andy channclr fl and P ate the phase errors of they pulse channel and of the reference of the Phil sensitive detector. The -ichannel phase error is zero per delimition. Adjusting for (1’) = (4) nulliliaa. subsequent adjusting for (2) = (3) which then wiU beequal to (1) and (4) nulliCes ,8. Note the sinedependena of (I ). . . . , (4) on u and B when wgt is chosen 10 lx a 90’.
10.
U. Haeberlerr and II. Kohlschfitrer, Proton sbielaing anisokpies
.frequency on resonance should be zero which is usually not the case: We accept beat frequencies up to about 300 Hz as satisfactory. The signal patterns ‘$tould behave equally when going off resonance both upfield and downfield. If they do not and/or the onresonance beat frequency is too high, the whole procedure is repeated. No extra corrections on the full four pulse cycle are made with the liquid sample. If we intend to work with the [ 1.3.2; l,y,,?l[ 1,3,2; 13,2nt sequence no further adjustments are needed. If we intend to work with the more standard 1 I ,3,21 sequence or its phase compensated version, 11,3,2lj [ 13,2]t, the nu tation angle of all pulses has to be increased somewhat in order to correct for finite pulse widths [ 141. This is done by increasing the out,put power of the transmitter until the highest resolution is obtained. For this procedure we do not know, unfortunately, about a material, which is as convenient for proton work as is CaF2 doped with paramag netic impurities for lgF work. We find that malonic acid and ferrocene are stilt the best choice. Both substances relax in a few minutes which is short enough for a search of the optimum nutation angle. This procedure leads more or less automatically to the correct scaling factor, especially, if any one of the phase compensated cycles is used. The same procedure applied to I9 F in CaF2 with BO along the [ 11 l] crystal axis leads consistently to a full linewidth at half height of about 100 Hz (170Hz scaled) for the [ 1,3,21 cycle and to about 70 Hz (145 Hz scaled)for the [1,3,2; l,T,21 I[ l,3,2; I,x2jt cycle.
Table 1 lists the compounds studied in this work. The oxalic, malonic and succinic acids are the first three members of saturated &carbonic acids. Up to the third member of the series at most two chemically different proton positions are encountered which is essential for keeping the spectra reasonably simple (see section 3). Oxalic acid is outstanding with respect to an NMR powder investigation inasmuch as aU itsproton; are equivalent. The NMR multiple pulse spectrum consists of a single powder pattern. The succinic acid anhydhde plays the same role f& aiphatic protons as does the oxalic acid for the carboxylic protons: all protons are equivalent (at
Table 1 Compouq&
inveaipted
in Lhir work
hnocene
least in an isolated molecule). Maleic acid and maleic acid anhydride form an acid-anhydride pair with oleftic protons. Fumaric acid is a stereoisomer to malcic acid and has been chosen for comparison purposes. Phthalic acid and its anhydride is an aromatic acidanhydride pair. The four aromatic protons in either compound are not equivalent by molecular symmetry, but we expect the differences in nuclear magnetic shielding to be well below our instrumental resolution. Ferrocene has been included in this study because in this compound ah protons are aromatic and equivalent_ Eking solid at room temperature, ferrocene is more convenient than benzene. All compounds were obtained from Merck Schuchard (Darmstadt, Germany). With the exception of oxalic acid they were investigated without futther purification. The sample .tubes with the various anhydrides were vacuum
Cl. Haeberlen and 0. Kohlsclriitter. Proton shielding anisotmpies
sealed in order to prevent hydrolysis*. Following Clarke and Davis [ 151 anhydrous oxalic acid was prepared by dissolving ox&c acid dihydrate in CCI,, refluxing for 1 hour, followed by distillation. This procedure was repeated 5 times. The solvent was then evaporated and the precipitate dried undervacuum in the presence of solid KOH. This procedure results in the formation of the orthorhombic (a-) modification of anhydrous oxalic acid [16]. This was confirmed by a powder X-ray analysis. A chemical analysis gave the following contents of carbon and hydrogen (theoretical values for (COOH), in brackets: C: 26.97% (26.68%); H: 2.33% (2.24%). Some of these compounds, especially the anhydrides, have very long spin-lattice relaxation times of up to several hours. Therefore we prepared six samples of every compound and placed them in the gap of our magnet outside the NMR probe. After a polarization period of, say, 20 hours we put them in!o the probe and obtained a single shot signal from each. The signals of the six samples were accumulated in the computer. All spectra have been recorded at room temperalure.
3. Results Fig. 2a shows a multiple pulse spectrum of anhydrous oxalic acid together with the resonance line of adamantane which is used as a reference throughout this work. The oxalic acid spectrum has the typical shape of a powder pattern for an axially symmetric shielding tensor. It was fitted with a theoretical pattern with both gaussian and lorcntzian component line broadening. Lorentzian broadening yielded a significantly better tit than gaussian broadening in accordance with our finding [ 131 that the multiple pulse line shape in CaF, is essentially lorentzian. The fit gave Au = 011 - ul= 17.8 ppm. The error limits are estimated to be -+I.5 ppm. According to the tit the component lines have a full width at half height of 260 Hz (unscaled).
* II was evident from the NMR mulliple pulse spectrum of our fust se1 of maleic acid anhydride samples that partial hydrolysis actually had taken place. This was the fust pnctical chemical analysis In a solid by the multiple pulse technique.
r
..
rdamanlane JJ
. _ .
.I\ _
.
:
-
:
.:. ._**
II
,.-i ..‘q .~......_.._. .*....II cl0
i.. al I,
0
-10 :
. .
A.
. . .
l lO.
-
:
,
0 -10 upf;e/d
_--._._._ ....._._. .._.. I -30
-20
-LO ppn
b) oxalic acid dihydrote
. .. -.
._._._.... :--.
a) onhydrous oxalic acid
.‘-..._
( -20
-._
__ -.
1
; ...-....;‘._ ._.. -30
-LO ppm
Fig. 2. Multiple pulse spectra of anhydrous
oxalic acid (a) and oxalic acid dihydrate (b). Also shown is the multiple pulse resonance line of adamantane. which is used as a reference. 0, and u1 as indicated in (a) are results of a computer tit.
Fig. 2b shows a multiple pulse spectrum of oxalic acid dihydrate. It again resembles an axially symmetric shielding tensor powder pattern, this is, however, quite fictitious: the main peak in the spectrum is due to the water protons. According to Yeung [Et] the shielding anisotropy of the water protons is quite small (= 7 ppm). The shoulder of the spectrum in fig. 2b must therefore be due to the carboxyl protons and we may conclude that one shielding component is shifted by as much as 28 ppm (downfield) from adamantane in accordance with Yeung’s data. Tire corresponding number for anhydrous oxalic acid is only 19 ppm. Figs. 3a, b and c show multiple pulse spectra of malonic acid, succinic acid and succinic acid arrhy dride. The main peaks on the left hand side of the spectra are from the aliphatic pro tons, the lower (right hand) peaks of the upper two spectra are the powder patterns peaks of the carboxyl protons. The very fact that the carboxyl proton peak is much lower than the aliphatic proton peak in the malonic acid spectrum indicates that the shielding anisotropy is much larger for the former protons.
00
U. .Haebcrlen
and 0. KolrlsclrU tter. Ao:on
shielding anirotropies
I -.. ._
. :i
. .: __ ....... .
.-
-
a) malanic acid
. 1
0) fumaric acid ‘.---_
f_
.. . . . ~ .- .
.. -. .-. -._._ ... ..
C..
.
. b) malcic acid
-._/.-.. : .. /---. .
: . :
-
.-*-._._. .
c)succinic acid anhydrIde
: .-. ....--- 9 _... II
+lO -
-...... -..._.
a,
.
.
.
. .
-
I,
c) maleic acid anhydride
_ -.-.. -.-..___
.---.---..
-..z...
1
. .. .
__._.~.-.-
I
-10 0 upfield
-20
+I0
-30 Ppm -
Fig. 3. Multiple pulse spectra of malonic acid. succinic acid and succinic acid anhydride. referenced to adamantane. 00 and o1 in (c)ate results of a computer fit. The arrows in (a) and (b) indicate from right IO left our estimates for oL (carboxj.1). ol(aIiphatic). and ob(aliphatic).
0 -10 upfield
-20
-30
ppm
Fig. 4. Multiple pulse spectn of fuma-ic acid, maleic acid zad mzdcic acid anhydride. referenced to adamantane. The arrows in (a) and (b) indicate our estimates for ol(ctrboxyl). _.
Table 2 Summary of rcsulls. Auiso= ~isotropic (I) - oisorro ic (carboxyl) from high resolution NMR; solvent: dimethylsulphoxide-d6. x strnds for tiphatic, oletinic or aromatic. p is the Pull wtdth at half height of the powder pattern component lines 1Acg,pl [ppml
ATSO lppml oxalic acid dihydatc anhydrous oxalic acid malonic acid succinic acid succinic acid anhydride fumaric acid maleic acid rnaleic acid anhydride phthslic acid
o&carboxyl)
Ao(carboxyl)
Ippml
Lppml
[ppml
01 (x) Ippml
0, (x) [wml
Au(x) [wml
D WI
- 28
9.2 9.6
15 16
- 18.8’) -20 - 20
- 13) -2 -1
17.8”) I8 19
- 6.8 - 6.8
-0.8 + 1.8
+6 + 8.6
6.38 6.0
I2 14.6
- 20.8 - 23.4
-4 + 2.4
16.8 25.8
-7.1a) - 8.8 - 8.8
+ 3 1;) - 818 -8.8
+ 10.2a) = 0 =O
5.1
12.4
- 19.6
+ 2.4
22
- 8.8 -7.2(+)
-8.8 - 7.2(-)
=O negative
- 7.2(+)
- 7.2(-)
- 5.la)
- iOJ”)
negarive - 5.4”)
phthzdic acid inhydride
“)Oblaincd
ol(cxboxyl)
by computer fit.
26OJ3”)
284/3”)
136 X 3/J2
EL
II. Haebcrlen and U. Kolrlrcl~lifrer. Proron shielding aniwrropics
In order to evaluate the shielding anisotropy of the carboxyl protons at least semiquantitatively from spectra as shown in figs. 3a and b and also in figs. 4a and b. we make the assumption that the average shifts are the same in the crystalhzcd and dissolved states. We hurry to point out that this is a questionable assumption, but we still hope that the results will indicate correctly general trends. With Au&= u&x) - a,(carboxyl) and Acpp= separation of the center of gravity of the x proton peak from the carboxyl proton powder pattern peak we obtain Au(carboxy1) = 3(A,,,-
Aom),
(1)
where x stands for aliphatic. aromatic or olefinic. Aa& was measured for the various acids using a 100 MHz Varian high resolution spectrometer and the results are listed in table 2. The peaks of the x protons are prominent enough to allow a reasonable estimation of the centers of gravity without interference from the carboxyl spectral contributions. Eq. (1) gives Au= +l8 ppm and +19 ppm for the carboxyl protons of the malonic and succinic acids, respectively. The difference is well within the error limits.
From lIgs. 3a-c one may infer that the anisotropy of the nuclear shielding of the aliphatic protons in this series is positive. The asymmetry of the shape is just noticeable in the malocic acid spectrum, distinct in the succinic acid spectrum and it allows a fairly accurate determination of Au for the succinic acid anhydride. Estimated values of ~1,011 and Au for the former two compounds and the results of a computer fit for the succinic acid anhydride are given in table 2. Figs. 4a-c show spectra of the fumaric and maleic acids and of maleic acid anhydride. A remarkable feature of the fumaric and maleic acid spectra is the shift of the respective carboxyl proton peaks. Au(carboxy1) as obtained from eq. (1) is 25.8 ppm and 16.8 ppm for the maleic and fumaric acids, respectively. We repeat that these numbers should be taken with great caution because of the assumption underlying eq. (I) and the difficulty to locate ul(carboxyl). Nevertheless we feel that the dilfcrence of the two shielding anise tropics is real. The sharpness and the symmetry of the olefmic proton peaks, especially in the maleic acid anhydride, reflect a very small shielding anisotropy in these positions: it is below the present resolution of our instrument.
krrocene
:.
. . .
J
.
.
-20
-_ 0,
I
a vsptc=vo
dvspc svo
..’ _‘.“-
:..
11 -10
Fig. 5. Multiple
0,
--+ . . .._._
I
1 +
.-._
-.
oppm
.-
a
pulse spccrr~of ferrocene
.
..
a,
4
II
‘-._ .A__
a
I
-IO
-2Uppm
with the Armor
frcqucncy v. both lower (a) and higher 0~) than the frequency of the speclromctcr (uspcc= 90.000 Mllz). a,, and aI are locltcd according IO computer fits. These speclrnwere obtained with the 81.3.2; l.T,Zl[
1.3,2; I.T2It
cycle.
The ph thalic acid and phthalic acid anhydride spcctra (not shown) indicate a small negative shielding anisotropy of the aromatic protons. The negative shielding anisotropy of aromatic protons becomes somewhat more evident in ferrocene (figs. 5a, b). The fcrrocene spectra were also subject to a computer lit. Both spectra yielded the same results. Table 2 summarizes the experimental results.
4. Discussion Although our experimental data are limited at present some general conclusions can be deduced from the results collected in table 2. If one classifies the values of Au according to magnitude and sign, one finds that carboxylic, aliphatic, olefmic and aromatic protons belong to different classes. Therefore it seems that Au for protons at least qualitatively reflects the immediate electronic environment of the protons under study. (a) Carboxylicprotom. The shift anisotropy is posi-
tive and quite large (ho z t I8 ppm). WCrecall that isotropic proton shifts fall within a range of not more than 13 ppm with very few exceptions. In spite of hydrogen bonding Au seems to be predominantly a molecular property: the center of gravity of the multiple pulse powder spectrum of anhydrous oxalic acid is shifted with respect to dissolved oxalic acid by only 0.6 ppm and the same holds for the other compounds as f2r as we can tell from our spectra.
82 ‘.
U. Haeberlen and U. Kohlschitter.
Nevertheless Au(carboxy1) does seem to be af.fccted to a measurable extent by the particular type. of hydrogen b&ding. In the compounds studied by US three different types of hydrogen bonding may be distinguished. (i) Intenole.cular hydrogen bonds between acid molecules which lead to the formation of infinite chains. This type of hydrogen bonding is encountered in the a-modification of anhydrous oxalic acid and In the malonic, succinic and phthalic acids [17-191. (ii) Intramolecular hydrogen bonding. In maleic acid one of the hydrogen bonds is intramolecular and leads to the formation of a ring [20] (see structure indicated in table I). (iii) Hydrogen bonds involving water molecules. Such bonds are encountered in oxalic acid dihydrate
WIItis evident from.table 2 that ;he shielding anisetropies of the carboxyi protons in group (i) rubstances arc all very similar, whereas ma&c acid and oxalic acid dihydrate, which belong to groups (ii) and (iii) form two distinct exceptions. More information about the influence of hydrogen bond-formation on the nuclear shielding anisotropy will (hopefully) be obtained from single crystal studies which not only yield the principal components ofo but also the orientation of the shielding principal axes system with respect to the crystal axes. Such studies are now in progress in our laboratory*. It must be pointed out, however, that quite often even complete data sets from single crystals are not sufficient to determine uniquely the shielding tensors 8 of the various nuclei relative to a molecular frame (cf. refs. [6,8,22], where the fmal assignments, though doubtlesrly correct, cannot be based purely and uniquely on measurements). in the one case known to us where a proton single crystal study has been performed [8], no simple and obvious relation between the nuclear shielding principal axes system and the crystal structure could be obtained. (b) Aliphdic proto~~s In all of our three examples Au is positive and it increases from malonic acid to succinic acid and to succinic acid anhydride. This trend,
l
Note added In proof: A single crystal study of malonic acid has been completed now. The hydrogen bond direction is the most shielded axis.
Proton shielding anisotropies
al least from nialonic to succinic acid, Seems to reflect
the well known decrease in the isotropic shielding for CH,X protons, as the electronegativity of.X is increased. (c) Olefinic protons In all our examples Ao is below detectability, which means, at least for maleic acid anhydride, lAoI< 2 ppm. The crystal structures of the maleic and succinic acid anhydtides are isomorphous [23]. The striking difference in their multiple pulse spectra is evidence that the nucle2.r shielding of covalently bound protons in the crystalline state is predominantly a molecular property. (d) Aromatic profons Our evidence is that the shielding anisotropy is negative. The five member fings of ferrocene, for which protons we obtained the clearest results, rotate rapidly at room temperature @C z lo-l1 set). This was established by measuring the temperature dependence of the proton spin-lattice relax&on time which, near room temperature, decreab ses with decreasing temperature with an apparent activation energy of 1.9 kcallmole. Our value for Ao(-5.4 ppm) therefore applies to the difference in nuclear magnetic shielding parallel and perpendicular to the C5 axes of the ring. The latter shielding is a motionally averaged property. The spectra of lgF in C,F, with and without motional averaging turned out to be the same which proved that (i) the shielding tensor in question is axially symmetric and that (ii) the shielding symmetry axis coincides with the rotation (= C,) axis of the molecule [II]. Low temperature measurements are needed to see whether the same is true for ferrocene. Ey partially orienting benzene in a nematic solvent, Englert [9] obtained Au(proton; benzene) =- 2.9.. . - 3.9 ppm where the perpendicular component again is motionally averaged. This result fits well into our suggested relationship between Au and the bonding situation. We now turn to the question of correlating our results WiLh theoretical approaches to the size and orientation of nuclear magnetic shielding tensors. It is well known that u can be separated into two terms, i.e., u= eP+ud which are called the paramagnetic ‘and diamagnetic terms, respectively. The latter prsvides for a shielding ( 05 > 0), the former for an antishielding (UP,, < 0) of the nucIeus. It seems that the diamagnetic teim can be handled quite well and reasonably simple by the techniques developed by
flygare and his coworkers [IO]. The problem of cal-
U. Haeberlen
and
U. Kohlschiirret.
culating up, on the other hand, is much more complicated, since excited electronic states arc involved. The difficulty of calculating up can be bypassed by exploiting the relationship between up and the spin rotation interaction tensor C [24,25] which can be measured in reasonably simple molecules by molecular beam techniques. Unfortunately, however, it turns out that for hydrogen I#&( and lo&[ arc often almost equal in magnitude, i.e., cris the small difference of two large terms. The consequence is that the calculations of 6 for protons are highly susceptible to errors in calculated values of o$ and to experimental errors in the determination 01 C (cr and fl indicate tensor components). Proton spin rotation interaction tensors for tbc molecules studied in this work are not available. As we have seen, however, our data suggest a general relation (maybe very crude only) between Au and the bonding situation near the hydrogen nucleus. We may therefore take H,O, CH, and CH,O, for which the spin rotation interaction constanrs are known, as representatives for protons bound to oxygen and to aliphatic and olefmic carbons. Gierke and Flygare [lo] have evaluated the diagonal shielding elements of the protons in Hz0 and CH,O in the inertial principal axes systems in the manner outlined above. Because the proton nuclear shielding and the inertial principal axes systems do not coincide in these molecules (unless accidentally) it is not possible to evaluate Au from their numbers. These imply, however, lAoI> 20 ppm for HI0 and Au = 7 ppm for CH,O. The extra difficulty with the different principal axes systems does not arise with CH, for which Anderson and Ramsey [26] have measured the relevant components of the proton spin rotation interaction tensor. Using their results and the reduced atom dipole moments given in ref. [27] we obtain by combining eqs. (2) and (14) of ref. [IO] Au(proton,
CH,)=
+12.6 ppm.
The uncertainty is as lame as 28.5 ppm taking into account only the quoted error limits of the spin rotation interaction components and disregarding completely all other error sources! Comparing the theoretical with our experimental results we find a rough correspondence in magnitude and, maybe more important, the same trend of de-
Proton shieldinK
83
anisonopies
creasing sh/clding anisotropies from oxygen bound to aliphatic and o!efinic protons. It will be interesting to see whether further experiments will confirm this trend and, if so, whether it will be possible to refine it more and more.
Acknowledgements
We are most grateful to Dr. N.B. Chanh of the bboratoirc de Cristallogaphie et de Physique Cristalline ?I 1’Universitb de Bordeaux for performing the X-ray analysis of our sample of anhydrous oxalic acid. Many of our colleagues have actively cofitributed to this work. A. Doehring has written the programs for the 100 10 and 10020 computers, J. Kcmpf did the line shape fits, H. Zimmerrnann prepared the samples. R. Grosescu measured T, of fcrrocene and P. Mansfield participated in part of the measurements. We gntefully acknowledge their help. We furthermore had the pleasure to discuss our results with Professor J. Jonas_ Our special thanks are due to H.W. Spiess for his continuous support of this work from the planning to the final writing stages. A grant for the apparatus from the Dcutschc Forschungsgcmcinschaft acknowledged.
is also gratefully
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84.
-[12]
U. Haeberlen
. P. h&&d.
J. Phys. C4 (1971)
and.U.
kohkchiitrer,
1444..
1131P. Mnn&icld.and U. Haeberlen, 2. Naturforsch.. [Id] hf-.hfchrir& 2. Naturforsch: 27a (1972) 1934.
[21]
i” pet:
1151 H.T. Clarke aqd A.W. Davis, Org. Synth. 1 (1921) 67.., 2. KxiitaUog. 91 (1935) 48. [17] E.G. Cox. M.W. Dougill and G.A. Jeffrey. J. Chem. tic. (1952 Iv) 4854. iIS] J.A. Goedkoop and C.H. MacCillawy. Acta Crysi. 10 (1957) 125. [19] J.D. Morrison and J.M. Roberlson. J. Chem. Sot. (1949 11) 980. 1201 M. Shahat. Acta Crygt. 5 (1952) 763.
: [16] S.B. Hendricks,
Proton tiierdim
anisorropies
L. Pauling, The nature of the chemical bond iCornell University Pros, 1960). (221 S. Pawk, A. Pines and J.S.. Wmgh, J. Chem. Phys., in, print. [231 M. Shahat. Proc. Phatmaceuti& Sot. of Egypt (Caho). 35 (1953) 57. (241 N.F. Ramsey. Phys. Rev. 78 (1950) 699. 1251 W.H. Flypre. J. Chem. Phys. 41 (1964) 793. I261 C.H. Anderson and N.F. Ramsey, Phys. Rev. 149 (1966) 14. [271’T.D.Gierke. H.L.T~claar and W.H. Flyme, J. Am. Chem. Sot. 94 (1972) 330.
PIIYSICS
2 (1973) 7684..@
NORTH-HOLLAND
PUBLISHING
COMPANY
‘,
NUCLEAR MAGNETIC SHIELDING ANlSOTROPrES FROM PROTON MULTIPLE PULSE SPECTRA. U. HAEBERLEN and U. KOHlSCHtiTTER Mar-Ptanck-lnstihrt
JLtirmedtiinische
Forschrmng, A breilungftir Received
molekulore Physik. 69 Heidelbrrg.
Germany
17 May 1973
Prolon multiple pulr speclra of powder samples of anhydrous osalic acid, oxalic acid dihydrate. malonic-. succinic-, phthalic-. maleic- and fumaric acid. succinic-. phthalic- and maleic acid anhydride, and fcrroccne have been recorded 31 90 hltlz. The spccln arc analysed in terms of the nuclear magnetic shielding ankorropy do =a~ - ul. No evidence for non-axhlly symmetric shielding has been obtained. The shielding anisouopics seem to reflect PI least qualitatively the immcdiale electronic environment of the protons under study. The anisotropies obtained for carboxylic, aliphalic. olefinic and aromatic protons arc = +lS, J +lO, = * 0 and 3 -5 ppm with substantial varialions among the various compounds studied.
1. Introduction Soon after the discovery of the phenomenon of nuclear magnetic shielding or “chemical shift” by Proctor in 1950 [l] a rapidly increasing flow of chemical shift determinations.
mostly
of protons,
set in. Apart
from a very few exceptions [2], however, only the isolropic part or the trace of the nuclear magnetic shielding tensor was accessible to measurement. With the advent of new techniques in recent years - oriep tation of molecules in nematic solvents [3), multiple pulse sequences [4], double resonance techniques [5] - and the availability of homogeneous high magnetic fields [6] there is now a similarly increasing flow of data about the anisoiropy of ‘he nuclear magnetic shielding. However, the experimental informa:ion now available is mostly about 19F and 13C, to a lesser extent about 31P, while 1H is definitely lagging behind. In 1969,the MlT group ran a few proton multiple pulse spectra [7] but to our knowledge only one serious NMR multiple pulse study has been undertaken since then on protons [8]. There are a few proton NMR investigations of molecules oriented in neniatic solvents [3,9], but these suffer from a general shortage of information as discussed in ref. [ 1 I]. This situation is caused by the fact that the isotropik as well as anisotropic nuclear magnetic shielding is usu,ally much smaller for hydrogen than .for all other
elements, whereas the (homogeneous) dipolar.coupling of the nuclei which tends to mask the anisotropy of the shielding is strongest for protons. We have applied multiple pulse methods to a number of powder samples of dicalbonic acids and their hydrides. All rtsults can be represented by axially symmetric shielding tensorsa. Au = u,, - uI is found to be positive and comparative!y large for carboxyl protons (== +I8 ppm), smaller but with a quite large variation for ahphatic protons (+6 ppm to +I0 ppm). below resolution (2 ppm) for olefinic protons, and negative for aromatic protons (z -5 ppm).
2. Experimental 2. I. A pparahrs We used a modified 90 MHz Bruker pulse spectra meter for this work. The power amplifier and the receiver were specially trimmed for the requirements of multiple pulse experiments. The pulse power available is in excess of 2.5 kW. Owing to the electronically regulated high voltage.power supply (Bruker) there is no noticeable droop during long pulse trains. Pulse rise and falltimes are about 100 nsec (10% cf 90%). We usually place a 6 dB attenuator between the transmitter and the (homemade) probe. This helps
II. Haeberlen and U. Kohlschiittcr.Proton shielding
greatly to reduce ringing between the tuned tank circuit of the power amplifier and the probe which itself has a bandwidth of 10 MHz. With the 6 dB attcnuator in the line, 90” pulses are about 700 nsec and the nuclear signal is faithfully detected 1.I3WC after the beginning of the rf pulses. After digitization (Bunker Ramo. Model BRSSO) the data are transferred via the direct access to memory to a general purpose ccmputer (Model 10010, C.I.I., Paris) where they are stored and accumulated, if necessary. After minor manipulations in the 10010, the data are transferrcd to the disc of a (larger) 10020 computer (C.I.I.) of which the 10010 is a satellite. All further data processing including Fourier transformation is done in the 10020 under control of the 10010. Data can be called from the disc to be displayed on a large CRT screen near the NMR spectrometer. The availability of on line Fourier transformation and immediate display of time and frequency data is essential for this type of experiments. In our spectrometer, the timing of the pulse SCquences is derived from the 10 MHz quartz oscillator of a 100 MHz frequency synthesizer (Schomandl). In order to maintain
full coherence between pulse edges ar.d rf, we work at 90.000 MHz. Consequently we must provide for a means to monitor the dc magnetic field, preferably in a precisely controlled manner. An external NMR field lock (Bruker) referenced by a fre-
anisolropik
77
width effects, phase glitches and small phase misalignments [ 131. Throughout this investigation r was chosen to bc 4 WC. With the output power of the transmitter some, what reduced from maximum (reduced Screen voltage), we iint adjust the width of the 90” pulses in the LX. +y channels by observing the NMR response of a liquid sample to a sequence of nominally 90” pulses. We then adjust the --x channel phase 180” off the x channel phase which is fwed. This is done by applying a (pX - T-f’_,-T) n sequence to the liquid sample. The criterion is a vanishing beat in the response. The most critical adjustments
are the phases of the *_v channels which are to be in quadrature to the +x channels. Usually one has to rely on the adjustment of a maximum NMR signal, which varies as cos 0. fl is the phase misalignment. This does not provide for a sharp criterion for 0. In order to obtain a better criterion, which actually has the sensitivity of a sin /3 dependence, we adopted the procedure $E”scribed in the legend of fig. I. After, say, they channel phase has been brought in quadrature to the x channel phase, the --y channel phase is made oppcsite to they channel phase in the same way as described for the ti channels. If now the pulses in all four channels are switched on to give the [ 1.3,21 sequence the observed beat
quency synthesizer, serves for that purpose. It turned out that the main output of the Schomandl synthesizer contains a lot of sidebands which interfere with the NMR signal and show up as a 1 MHz, 100 kHz and 10 kHz ripple on the NMR signal. The baseline is free of this ripple. In order to circumvent this problem, we now use as the basic frequency source of the spectrometer a times 9 frequency multiplier, which is also driven by the buffcred 10 MHz quartz oscillator and which is followed by a sharp bandpass filter. 2.2. Alignment procedure For the earlier part of this work we used the WAHUHA four pulse or [ 1,3,20 cycle in the notation introduced by Mansfield [ 121. The more recent spectra were obtained with the [1,3,2; 1,7,21 [ 1,3,2; 1,7,2]lt sixteen pulse cycle which is self-compensating with respect to rf inhomogeneity, finite pulse
Fig. i. Scheme to adjust quadrature phases in two pulse channels. cg.. the-x andy channclr fl and P ate the phase errors of they pulse channel and of the reference of the Phil sensitive detector. The -ichannel phase error is zero per delimition. Adjusting for (1’) = (4) nulliliaa. subsequent adjusting for (2) = (3) which then wiU beequal to (1) and (4) nulliCes ,8. Note the sinedependena of (I ). . . . , (4) on u and B when wgt is chosen 10 lx a 90’.
10.
U. Haeberlerr and II. Kohlschfitrer, Proton sbielaing anisokpies
.frequency on resonance should be zero which is usually not the case: We accept beat frequencies up to about 300 Hz as satisfactory. The signal patterns ‘$tould behave equally when going off resonance both upfield and downfield. If they do not and/or the onresonance beat frequency is too high, the whole procedure is repeated. No extra corrections on the full four pulse cycle are made with the liquid sample. If we intend to work with the [ 1.3.2; l,y,,?l[ 1,3,2; 13,2nt sequence no further adjustments are needed. If we intend to work with the more standard 1 I ,3,21 sequence or its phase compensated version, 11,3,2lj [ 13,2]t, the nu tation angle of all pulses has to be increased somewhat in order to correct for finite pulse widths [ 141. This is done by increasing the out,put power of the transmitter until the highest resolution is obtained. For this procedure we do not know, unfortunately, about a material, which is as convenient for proton work as is CaF2 doped with paramag netic impurities for lgF work. We find that malonic acid and ferrocene are stilt the best choice. Both substances relax in a few minutes which is short enough for a search of the optimum nutation angle. This procedure leads more or less automatically to the correct scaling factor, especially, if any one of the phase compensated cycles is used. The same procedure applied to I9 F in CaF2 with BO along the [ 11 l] crystal axis leads consistently to a full linewidth at half height of about 100 Hz (170Hz scaled) for the [ 1,3,21 cycle and to about 70 Hz (145 Hz scaled)for the [1,3,2; l,T,21 I[ l,3,2; I,x2jt cycle.
Table 1 lists the compounds studied in this work. The oxalic, malonic and succinic acids are the first three members of saturated &carbonic acids. Up to the third member of the series at most two chemically different proton positions are encountered which is essential for keeping the spectra reasonably simple (see section 3). Oxalic acid is outstanding with respect to an NMR powder investigation inasmuch as aU itsproton; are equivalent. The NMR multiple pulse spectrum consists of a single powder pattern. The succinic acid anhydhde plays the same role f& aiphatic protons as does the oxalic acid for the carboxylic protons: all protons are equivalent (at
Table 1 Compouq&
inveaipted
in Lhir work
hnocene
least in an isolated molecule). Maleic acid and maleic acid anhydride form an acid-anhydride pair with oleftic protons. Fumaric acid is a stereoisomer to malcic acid and has been chosen for comparison purposes. Phthalic acid and its anhydride is an aromatic acidanhydride pair. The four aromatic protons in either compound are not equivalent by molecular symmetry, but we expect the differences in nuclear magnetic shielding to be well below our instrumental resolution. Ferrocene has been included in this study because in this compound ah protons are aromatic and equivalent_ Eking solid at room temperature, ferrocene is more convenient than benzene. All compounds were obtained from Merck Schuchard (Darmstadt, Germany). With the exception of oxalic acid they were investigated without futther purification. The sample .tubes with the various anhydrides were vacuum
Cl. Haeberlen and 0. Kohlsclriitter. Proton shielding anisotmpies
sealed in order to prevent hydrolysis*. Following Clarke and Davis [ 151 anhydrous oxalic acid was prepared by dissolving ox&c acid dihydrate in CCI,, refluxing for 1 hour, followed by distillation. This procedure was repeated 5 times. The solvent was then evaporated and the precipitate dried undervacuum in the presence of solid KOH. This procedure results in the formation of the orthorhombic (a-) modification of anhydrous oxalic acid [16]. This was confirmed by a powder X-ray analysis. A chemical analysis gave the following contents of carbon and hydrogen (theoretical values for (COOH), in brackets: C: 26.97% (26.68%); H: 2.33% (2.24%). Some of these compounds, especially the anhydrides, have very long spin-lattice relaxation times of up to several hours. Therefore we prepared six samples of every compound and placed them in the gap of our magnet outside the NMR probe. After a polarization period of, say, 20 hours we put them in!o the probe and obtained a single shot signal from each. The signals of the six samples were accumulated in the computer. All spectra have been recorded at room temperalure.
3. Results Fig. 2a shows a multiple pulse spectrum of anhydrous oxalic acid together with the resonance line of adamantane which is used as a reference throughout this work. The oxalic acid spectrum has the typical shape of a powder pattern for an axially symmetric shielding tensor. It was fitted with a theoretical pattern with both gaussian and lorcntzian component line broadening. Lorentzian broadening yielded a significantly better tit than gaussian broadening in accordance with our finding [ 131 that the multiple pulse line shape in CaF, is essentially lorentzian. The fit gave Au = 011 - ul= 17.8 ppm. The error limits are estimated to be -+I.5 ppm. According to the tit the component lines have a full width at half height of 260 Hz (unscaled).
* II was evident from the NMR mulliple pulse spectrum of our fust se1 of maleic acid anhydride samples that partial hydrolysis actually had taken place. This was the fust pnctical chemical analysis In a solid by the multiple pulse technique.
r
..
rdamanlane JJ
. _ .
.I\ _
.
:
-
:
.:. ._**
II
,.-i ..‘q .~......_.._. .*....II cl0
i.. al I,
0
-10 :
. .
A.
. . .
l lO.
-
:
,
0 -10 upf;e/d
_--._._._ ....._._. .._.. I -30
-20
-LO ppn
b) oxalic acid dihydrote
. .. -.
._._._.... :--.
a) onhydrous oxalic acid
.‘-..._
( -20
-._
__ -.
1
; ...-....;‘._ ._.. -30
-LO ppm
Fig. 2. Multiple pulse spectra of anhydrous
oxalic acid (a) and oxalic acid dihydrate (b). Also shown is the multiple pulse resonance line of adamantane. which is used as a reference. 0, and u1 as indicated in (a) are results of a computer tit.
Fig. 2b shows a multiple pulse spectrum of oxalic acid dihydrate. It again resembles an axially symmetric shielding tensor powder pattern, this is, however, quite fictitious: the main peak in the spectrum is due to the water protons. According to Yeung [Et] the shielding anisotropy of the water protons is quite small (= 7 ppm). The shoulder of the spectrum in fig. 2b must therefore be due to the carboxyl protons and we may conclude that one shielding component is shifted by as much as 28 ppm (downfield) from adamantane in accordance with Yeung’s data. Tire corresponding number for anhydrous oxalic acid is only 19 ppm. Figs. 3a, b and c show multiple pulse spectra of malonic acid, succinic acid and succinic acid arrhy dride. The main peaks on the left hand side of the spectra are from the aliphatic pro tons, the lower (right hand) peaks of the upper two spectra are the powder patterns peaks of the carboxyl protons. The very fact that the carboxyl proton peak is much lower than the aliphatic proton peak in the malonic acid spectrum indicates that the shielding anisotropy is much larger for the former protons.
00
U. .Haebcrlen
and 0. KolrlsclrU tter. Ao:on
shielding anirotropies
I -.. ._
. :i
. .: __ ....... .
.-
-
a) malanic acid
. 1
0) fumaric acid ‘.---_
f_
.. . . . ~ .- .
.. -. .-. -._._ ... ..
C..
.
. b) malcic acid
-._/.-.. : .. /---. .
: . :
-
.-*-._._. .
c)succinic acid anhydrIde
: .-. ....--- 9 _... II
+lO -
-...... -..._.
a,
.
.
.
. .
-
I,
c) maleic acid anhydride
_ -.-.. -.-..___
.---.---..
-..z...
1
. .. .
__._.~.-.-
I
-10 0 upfield
-20
+I0
-30 Ppm -
Fig. 3. Multiple pulse spectra of malonic acid. succinic acid and succinic acid anhydride. referenced to adamantane. 00 and o1 in (c)ate results of a computer fit. The arrows in (a) and (b) indicate from right IO left our estimates for oL (carboxj.1). ol(aIiphatic). and ob(aliphatic).
0 -10 upfield
-20
-30
ppm
Fig. 4. Multiple pulse spectn of fuma-ic acid, maleic acid zad mzdcic acid anhydride. referenced to adamantane. The arrows in (a) and (b) indicate our estimates for ol(ctrboxyl). _.
Table 2 Summary of rcsulls. Auiso= ~isotropic (I) - oisorro ic (carboxyl) from high resolution NMR; solvent: dimethylsulphoxide-d6. x strnds for tiphatic, oletinic or aromatic. p is the Pull wtdth at half height of the powder pattern component lines 1Acg,pl [ppml
ATSO lppml oxalic acid dihydatc anhydrous oxalic acid malonic acid succinic acid succinic acid anhydride fumaric acid maleic acid rnaleic acid anhydride phthslic acid
o&carboxyl)
Ao(carboxyl)
Ippml
Lppml
[ppml
01 (x) Ippml
0, (x) [wml
Au(x) [wml
D WI
- 28
9.2 9.6
15 16
- 18.8’) -20 - 20
- 13) -2 -1
17.8”) I8 19
- 6.8 - 6.8
-0.8 + 1.8
+6 + 8.6
6.38 6.0
I2 14.6
- 20.8 - 23.4
-4 + 2.4
16.8 25.8
-7.1a) - 8.8 - 8.8
+ 3 1;) - 818 -8.8
+ 10.2a) = 0 =O
5.1
12.4
- 19.6
+ 2.4
22
- 8.8 -7.2(+)
-8.8 - 7.2(-)
=O negative
- 7.2(+)
- 7.2(-)
- 5.la)
- iOJ”)
negarive - 5.4”)
phthzdic acid inhydride
“)Oblaincd
ol(cxboxyl)
by computer fit.
26OJ3”)
284/3”)
136 X 3/J2
EL
II. Haebcrlen and U. Kolrlrcl~lifrer. Proron shielding aniwrropics
In order to evaluate the shielding anisotropy of the carboxyl protons at least semiquantitatively from spectra as shown in figs. 3a and b and also in figs. 4a and b. we make the assumption that the average shifts are the same in the crystalhzcd and dissolved states. We hurry to point out that this is a questionable assumption, but we still hope that the results will indicate correctly general trends. With Au&= u&x) - a,(carboxyl) and Acpp= separation of the center of gravity of the x proton peak from the carboxyl proton powder pattern peak we obtain Au(carboxy1) = 3(A,,,-
Aom),
(1)
where x stands for aliphatic. aromatic or olefinic. Aa& was measured for the various acids using a 100 MHz Varian high resolution spectrometer and the results are listed in table 2. The peaks of the x protons are prominent enough to allow a reasonable estimation of the centers of gravity without interference from the carboxyl spectral contributions. Eq. (1) gives Au= +l8 ppm and +19 ppm for the carboxyl protons of the malonic and succinic acids, respectively. The difference is well within the error limits.
From lIgs. 3a-c one may infer that the anisotropy of the nuclear shielding of the aliphatic protons in this series is positive. The asymmetry of the shape is just noticeable in the malocic acid spectrum, distinct in the succinic acid spectrum and it allows a fairly accurate determination of Au for the succinic acid anhydride. Estimated values of ~1,011 and Au for the former two compounds and the results of a computer fit for the succinic acid anhydride are given in table 2. Figs. 4a-c show spectra of the fumaric and maleic acids and of maleic acid anhydride. A remarkable feature of the fumaric and maleic acid spectra is the shift of the respective carboxyl proton peaks. Au(carboxy1) as obtained from eq. (1) is 25.8 ppm and 16.8 ppm for the maleic and fumaric acids, respectively. We repeat that these numbers should be taken with great caution because of the assumption underlying eq. (I) and the difficulty to locate ul(carboxyl). Nevertheless we feel that the dilfcrence of the two shielding anise tropics is real. The sharpness and the symmetry of the olefmic proton peaks, especially in the maleic acid anhydride, reflect a very small shielding anisotropy in these positions: it is below the present resolution of our instrument.
krrocene
:.
. . .
J
.
.
-20
-_ 0,
I
a vsptc=vo
dvspc svo
..’ _‘.“-
:..
11 -10
Fig. 5. Multiple
0,
--+ . . .._._
I
1 +
.-._
-.
oppm
.-
a
pulse spccrr~of ferrocene
.
..
a,
4
II
‘-._ .A__
a
I
-IO
-2Uppm
with the Armor
frcqucncy v. both lower (a) and higher 0~) than the frequency of the speclromctcr (uspcc= 90.000 Mllz). a,, and aI are locltcd according IO computer fits. These speclrnwere obtained with the 81.3.2; l.T,Zl[
1.3,2; I.T2It
cycle.
The ph thalic acid and phthalic acid anhydride spcctra (not shown) indicate a small negative shielding anisotropy of the aromatic protons. The negative shielding anisotropy of aromatic protons becomes somewhat more evident in ferrocene (figs. 5a, b). The fcrrocene spectra were also subject to a computer lit. Both spectra yielded the same results. Table 2 summarizes the experimental results.
4. Discussion Although our experimental data are limited at present some general conclusions can be deduced from the results collected in table 2. If one classifies the values of Au according to magnitude and sign, one finds that carboxylic, aliphatic, olefmic and aromatic protons belong to different classes. Therefore it seems that Au for protons at least qualitatively reflects the immediate electronic environment of the protons under study. (a) Carboxylicprotom. The shift anisotropy is posi-
tive and quite large (ho z t I8 ppm). WCrecall that isotropic proton shifts fall within a range of not more than 13 ppm with very few exceptions. In spite of hydrogen bonding Au seems to be predominantly a molecular property: the center of gravity of the multiple pulse powder spectrum of anhydrous oxalic acid is shifted with respect to dissolved oxalic acid by only 0.6 ppm and the same holds for the other compounds as f2r as we can tell from our spectra.
82 ‘.
U. Haeberlen and U. Kohlschitter.
Nevertheless Au(carboxy1) does seem to be af.fccted to a measurable extent by the particular type. of hydrogen b&ding. In the compounds studied by US three different types of hydrogen bonding may be distinguished. (i) Intenole.cular hydrogen bonds between acid molecules which lead to the formation of infinite chains. This type of hydrogen bonding is encountered in the a-modification of anhydrous oxalic acid and In the malonic, succinic and phthalic acids [17-191. (ii) Intramolecular hydrogen bonding. In maleic acid one of the hydrogen bonds is intramolecular and leads to the formation of a ring [20] (see structure indicated in table I). (iii) Hydrogen bonds involving water molecules. Such bonds are encountered in oxalic acid dihydrate
WIItis evident from.table 2 that ;he shielding anisetropies of the carboxyi protons in group (i) rubstances arc all very similar, whereas ma&c acid and oxalic acid dihydrate, which belong to groups (ii) and (iii) form two distinct exceptions. More information about the influence of hydrogen bond-formation on the nuclear shielding anisotropy will (hopefully) be obtained from single crystal studies which not only yield the principal components ofo but also the orientation of the shielding principal axes system with respect to the crystal axes. Such studies are now in progress in our laboratory*. It must be pointed out, however, that quite often even complete data sets from single crystals are not sufficient to determine uniquely the shielding tensors 8 of the various nuclei relative to a molecular frame (cf. refs. [6,8,22], where the fmal assignments, though doubtlesrly correct, cannot be based purely and uniquely on measurements). in the one case known to us where a proton single crystal study has been performed [8], no simple and obvious relation between the nuclear shielding principal axes system and the crystal structure could be obtained. (b) Aliphdic proto~~s In all of our three examples Au is positive and it increases from malonic acid to succinic acid and to succinic acid anhydride. This trend,
l
Note added In proof: A single crystal study of malonic acid has been completed now. The hydrogen bond direction is the most shielded axis.
Proton shielding anisotropies
al least from nialonic to succinic acid, Seems to reflect
the well known decrease in the isotropic shielding for CH,X protons, as the electronegativity of.X is increased. (c) Olefinic protons In all our examples Ao is below detectability, which means, at least for maleic acid anhydride, lAoI< 2 ppm. The crystal structures of the maleic and succinic acid anhydtides are isomorphous [23]. The striking difference in their multiple pulse spectra is evidence that the nucle2.r shielding of covalently bound protons in the crystalline state is predominantly a molecular property. (d) Aromatic profons Our evidence is that the shielding anisotropy is negative. The five member fings of ferrocene, for which protons we obtained the clearest results, rotate rapidly at room temperature @C z lo-l1 set). This was established by measuring the temperature dependence of the proton spin-lattice relax&on time which, near room temperature, decreab ses with decreasing temperature with an apparent activation energy of 1.9 kcallmole. Our value for Ao(-5.4 ppm) therefore applies to the difference in nuclear magnetic shielding parallel and perpendicular to the C5 axes of the ring. The latter shielding is a motionally averaged property. The spectra of lgF in C,F, with and without motional averaging turned out to be the same which proved that (i) the shielding tensor in question is axially symmetric and that (ii) the shielding symmetry axis coincides with the rotation (= C,) axis of the molecule [II]. Low temperature measurements are needed to see whether the same is true for ferrocene. Ey partially orienting benzene in a nematic solvent, Englert [9] obtained Au(proton; benzene) =- 2.9.. . - 3.9 ppm where the perpendicular component again is motionally averaged. This result fits well into our suggested relationship between Au and the bonding situation. We now turn to the question of correlating our results WiLh theoretical approaches to the size and orientation of nuclear magnetic shielding tensors. It is well known that u can be separated into two terms, i.e., u= eP+ud which are called the paramagnetic ‘and diamagnetic terms, respectively. The latter prsvides for a shielding ( 05 > 0), the former for an antishielding (UP,, < 0) of the nucIeus. It seems that the diamagnetic teim can be handled quite well and reasonably simple by the techniques developed by
flygare and his coworkers [IO]. The problem of cal-
U. Haeberlen
and
U. Kohlschiirret.
culating up, on the other hand, is much more complicated, since excited electronic states arc involved. The difficulty of calculating up can be bypassed by exploiting the relationship between up and the spin rotation interaction tensor C [24,25] which can be measured in reasonably simple molecules by molecular beam techniques. Unfortunately, however, it turns out that for hydrogen I#&( and lo&[ arc often almost equal in magnitude, i.e., cris the small difference of two large terms. The consequence is that the calculations of 6 for protons are highly susceptible to errors in calculated values of o$ and to experimental errors in the determination 01 C (cr and fl indicate tensor components). Proton spin rotation interaction tensors for tbc molecules studied in this work are not available. As we have seen, however, our data suggest a general relation (maybe very crude only) between Au and the bonding situation near the hydrogen nucleus. We may therefore take H,O, CH, and CH,O, for which the spin rotation interaction constanrs are known, as representatives for protons bound to oxygen and to aliphatic and olefmic carbons. Gierke and Flygare [lo] have evaluated the diagonal shielding elements of the protons in Hz0 and CH,O in the inertial principal axes systems in the manner outlined above. Because the proton nuclear shielding and the inertial principal axes systems do not coincide in these molecules (unless accidentally) it is not possible to evaluate Au from their numbers. These imply, however, lAoI> 20 ppm for HI0 and Au = 7 ppm for CH,O. The extra difficulty with the different principal axes systems does not arise with CH, for which Anderson and Ramsey [26] have measured the relevant components of the proton spin rotation interaction tensor. Using their results and the reduced atom dipole moments given in ref. [27] we obtain by combining eqs. (2) and (14) of ref. [IO] Au(proton,
CH,)=
+12.6 ppm.
The uncertainty is as lame as 28.5 ppm taking into account only the quoted error limits of the spin rotation interaction components and disregarding completely all other error sources! Comparing the theoretical with our experimental results we find a rough correspondence in magnitude and, maybe more important, the same trend of de-
Proton shieldinK
83
anisonopies
creasing sh/clding anisotropies from oxygen bound to aliphatic and o!efinic protons. It will be interesting to see whether further experiments will confirm this trend and, if so, whether it will be possible to refine it more and more.
Acknowledgements
We are most grateful to Dr. N.B. Chanh of the bboratoirc de Cristallogaphie et de Physique Cristalline ?I 1’Universitb de Bordeaux for performing the X-ray analysis of our sample of anhydrous oxalic acid. Many of our colleagues have actively cofitributed to this work. A. Doehring has written the programs for the 100 10 and 10020 computers, J. Kcmpf did the line shape fits, H. Zimmerrnann prepared the samples. R. Grosescu measured T, of fcrrocene and P. Mansfield participated in part of the measurements. We gntefully acknowledge their help. We furthermore had the pleasure to discuss our results with Professor J. Jonas_ Our special thanks are due to H.W. Spiess for his continuous support of this work from the planning to the final writing stages. A grant for the apparatus from the Dcutschc Forschungsgcmcinschaft acknowledged.
is also gratefully
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