Simplification of Thiol Nuclear Magnetic Resonance Spectra by in Situ Derivatization SIR: It has recently been reported that alcohols and glycols may be characterized by in situ derivatization with reactive ketenes or isocyanates (I). Such treatment produces paramagnetic shifts of protons alpha to the hydroxy group. The magnitude of the shift, the multiplicity of the shifted peaks, and their relative areas were found to be valuable aids in determining the structure of the parent alcohol. We wish to report that we have extended this method to the in situ derivatization of alkyl and aryl, mono- and dithiols with trichloroacetyl isocyanate (Eastman Kodak, Distillation Products Industries). 0
0
RSH
+ CI3CCNCO
+
d
I
I
C l 3 CCONHCOSCH2CH2C02CH3 C13CCONHCOSCH2CH2C02CH3 a
c
b
C
0
/I
/I
HSCH2CH2CO2CH3 d a b c
I/
a
RSCNHCCCl3
J
I
L. . . I
RESULTS A N D DISCUSSION
Alkyl and aryl thiols may be rapidly and quantitatively derivatized by simply adding several drops of isocyanate reagent to the sample contained in an Nh'IR tube (the progress of this reaction may be conveniently followed by observing the disappearance of the thiol proton whose resonance position is usually in the 0.8-3.0 p.p.m. region (2). The derived monothiocarbamates (I) are readily soluble, at 107, concentration, in deuteriochloroform. Dimethylsulfoxide-& may be used as a solvent for the less soluble derivatives of dithiols. In situ derivatization of thiols simplifies the interpretation of their spectra in two ways. First, introduction of the highjy electronegative carbamate group induces a strong paramagnetic shift of the resonance positions of protons on the carbon alpha to the sulfur group. This technique not only has the potential of separating overlapping peaks, a problem which is more serious in the spectra of thiols than in alcohols (protons on carbon atoms alpha to oxygen are normally in the 3 . 5 4 . 5 p.p.m. range already), but it converts higher order AB type systems, formed with protons on carbons beta to sulfur, to first order AX systems more amenable to simple analysis. The chemical shifts for alpha protons in several representative samples are given in Table I. Methylene protons adjacent to sulfur are shifted 0.45-0.55 p.1i.m. downfield in the derivatives. A shift of about 0.71 p.p.m. downfield for methine protons has been observed in the two cases studied. The magnitude of these shifts seems to be very specific
b
I
,
I
E
I..
I ,, *
.
*
9
I
I
. , ! .. . . . . .. . . I
4 I
I I
I
,
I
1
I
I
.
I
I
c
I
I
I
I I
I
. ... I
f
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I
Figure 1 . Nuclear magnetic resonance spectra of methyl 3-thiopropionate and its thiocarbamate derivative
and, in itself, good evidence for the nature of substitution on the alpha carbon. The second manner of spectral simplification by derivatization is the removal of coupling between the thiol proton and protons on the adjacent
I I
carbon atom (HCSH). Simplification of the resonance signal of alpha methylene or methine protons allows a more ready analysis of its multiplicity pattern and thus better determination of the nature of substitution on the alpha carbon. It may be noted that this feature is unique with thiols since rapid exchange of hydroxyl protons in alcohols
Table 1.
usually does not permit observation of HCOH coupling. Figure 1 shows the NMR spectrum of methyl 3-thiolpropionate and its thiocarbamate derivative. The four methylene protons of the parent compound form with the thiol proton an AMzNz second order system. The resonance peaks for the methylene protons (SCH2CH2CO-) are overlapped and appear as a four proton multiplet centered at about 2.63 p.p.m. The thiol proton is a multiplet, approximating a triplet, at 1.63 p.p.m. On admixture of excess trichloroacetyl isocyanate to the sample in the NMR sample tube, rapid and complete
a-Hydrogen Chemical Shifts for Thiols and Isocyanate Derivatives *Hydrogen chemical A p.p.m. shift, p.p.m.0 lo 2O
Compound CHFCHCHBH CH?C( CHs)CHBH CHaCH2CHzCHzSH (CH.LCHCH%qH CHaCH(SH)dH2CHa CHa(CH2)sCHzSH HSCHzCH(SH)CHs CHZ P U v--
Thiol
Derivative
Thiol
3.12 t 3.08 d 2.48 q 2.37 a2 2 . 8 4 hp 2.48 q 2.73 3.03 8 p 2.63 m 2.60 q 2.50 m
3.58 d 3.62 8 2.94 t 2.85 d 3 . 5 5 hz 2.93 t 3 . 2 2 dd 3.75 m 3.18 t 3.05 t 3.02 d
0.46
HSCHiCHzCOzCHa CHFCHCHBCHZCH&HBH CHFC( CHa )CHzSCHzCH(CH8)CHBH a Notation: s = singlet, d = doublet, t = triplet, q h p = heptet, dd = double doublet, m = multiplet.
=
Thiol
0.54 0.46 0.48 0.71
0.45 0.49
0.55 0.45 0.52 quadruplet, hz
VOL 38, NO. 10, SEPTEMBER 1966
0.72
=
hextet,
1407
conversion to the thiocarbamate was achieved. This reaction was followed by observing the complete disappearance of the thiol proton. The NMR spectrum of the derivative shows a downfield shift of 0.55 p.p.m. for the group to This group appears as a triplet at 3.18 P.P.m. ( J = 6.5 C.P.S.) coupled with the methylene group alpha to the carbonyl which also is a triplet at 2.70
p.p.m. The NH peak of the derivative is broad and appears a t about 10 p.p.m. EXPERIMENTAL
were recorded as mately 10% deuteriochloroform or dimethylsu~foxide-&solutions on a Varian Associates A-60 NMR spectrometer, Peak positions are given in parts per million (p.p.m.) downfield from internal tetramethylsilane a t 0.
LITERATURE CITED
( 1 ) Goodlett, V. W., ANAL. CHEM.37, 431 (1965). ( 2 ) Marcus, S. H., ILlitler, S. I., J . Phys. Chem. 68, 331 (1964).
PETERE. BUTLER
WOLFGANG H. AIUELLER ESSOResearch and Engineering Co. Linden, N. J.
Improved Spectral Measurements in Spark Source Mass Spectrometry Using Transmittance Areas SIR: One of the factors affecting quantitative results in spark source mass spectrography is the measurement of spectral intensities as recorded on a photographic plate. Preliminary results in a previous study (3) indicated that improved precision was obtained through the use of planimeter-measured transmittance peak areas rather than the more conventional absorbance peak height data. This study represents a more comprehensive investigation of the use of transmittance areas in the evaluation of mass spectrographic data. The determination of transmittance areas was greatly facilitated by the use of an electronic peak integrator installed as an auxiliary output on a conventional recording densitometer.
Figure 1.
Schematic diagram of the electronic peak integrator
EXPERIMENTAL
A Kuclide GRAF-2 spark source mass spectrograph and a Jarrell-Ash Model 23-100 recording microphotometer were used in this study. The experimental and instrumental parameters have been previously described ( 3 ) . These parameters are not of primary importance here because absorbances at the peak maxima were compared with integrated areas of the same peaks in transmittance units, both sets of data being obtained from the same photographic record in every instance. The Ilford &-I1 emulsion was used. Integrated Area Measurement. Transmittance areas were determined from the photographic data using a rapid, convenient electronic integrator system coupled to the densitometer. A schematic diagram of the integrator, designed and constructed to our speoifications by the DeVar Division of Consolidated Electrodynamics Corp., is presented in Figure 1. The modular unit consists entirely of solid-state components, each module performing a given function. The operation and description of the integrator is best accomplished by considering a peak being scanned by the densitometer. The background on one side of the peak is scanned with the external switch controlling the integrator in 1408
ANALYTICAL CHEMISTRY
the closed position. This background signal is introduced into two parallel systems, an adder-subtractor module, and an adjustable lag module. During this time, the adjustable lag determines the average value of the background and equal signals are being received by the two terminals of the addersubtractor. The signals being equal, there is no output from the latter module. As the scan reaches the vicinity of the peak to be integrated, the external switch is opened, thus interrupting the input to the lag module. Under this condition, the lag unit maintains an output equal to the average background. As the incoming peak signal changes, the signal received by the positive side of the addersubtractor increases, producing a difference in the two signals received by the latter. The value of this difference is continuously integrated, converted to square-wave pulses, and displayed on an electronic counter. Integration ends when the external switch is closed and the system immediately begins sampling to establish a new background average. The unit, as used in this laboratory, was set to average the background over 8 period of five time constants, the latter having been adjusted to 1 second.
The amplifier gain was set to produce 6OOO counts per minute per 10-mv. signal. Any further increase in the signal per unit area may be readily obtained by increasing the gain or by reducing the rate of the densitometer scan. Areas determined electronically were linear with planimeter-measured areas. Extensive usage of the integrator unit has demonstrated that the times of initiation and termination of integration are not critical provided, of course, that the times selected are not within the interval during which the peak signal is being received. The presence of an extraneous, unresolved line or band near that to be measured may, however, preclude accurate sampling of the background level. In instances where the peak is superposed on a sloping background, the area can be reproducibly measured by scanning from both directions and taking an average of the two results. To evaluate the precision with which an area could be determined, five replicate integrations of photographic data representing a wide range of intensity levels were run by two operators. The standard deviation calculated from these data was, in every instance, equal to or less than dr,where N
