13c cp-mas nmr studies of s-alkyl-1,4-dithianium salts: conformational motion in the solid state

13C CP-MAS NMR studies ofS-alkyl-1,4-dithianium salts:conformational motion in the solid state

George W. Wagnera,* , Yu-Chu Yangb

aGeo-Centers, Inc., Gunpowder Branch Box 68, Aberdeen Proving Ground, MD 21010-0068 USAbU.S. Army European Research Office, USARDSG-UK, PSC 802 Box 15, FPO AE 09499-1500 UK

Received 17 September 1998; accepted 20 November 1998

Abstract

S-(2-chloroethyl)-1,4-dithianium, a persistent solid degradation product of mustard and the major constituent of ‘‘mustardheels’’, exhibits motional line-broadening in its13C CP-MAS NMR spectrum. Variable temperature examination of thecompound and a series of relatedS-alkyl-1,4-dithianium and thianium salts demonstrates the generality of the motion inthese cyclic sulfonium salts. The underlying motional process for the carbons in the six-member rings cannot be attributedto ring rotation, the classical interpretation for cyclohexane, but is consistent with conformational motion. Solid dithiane, andthe low temperature phases of cyclohexane and thiane were also studied by variable temperature13C MAS NMR to contrast thebehavior of these neutral rings with the cyclic sulfonium salts. Distinguishing characteristics of the conformational mechanisminclude smallerEa’s (2.3–6.1 kcal/mol) and line-broadening (400–600 Hz), compared to the ring-rotation mechanism operativein solid dithiane (12.9 kcal/mol, 1300 Hz) and the low-temperature phase of cyclohexane (11.4 kcal/mol, 925 Hz). The valuesfor the low-temperature phase of thiane (3.8 kcal/mol, 525 Hz) are more consistent with conformational motion rather thanhindered-rotation.q 1999 Elsevier Science B.V. All rights reserved.

Keywords:13C CP-MAS NMR; Dithianium salts; Cyclic sulfonium ions; Conformational motion; Mustard

1. Introduction

The cyclic sulfonium ionS-(2-chloroethyl)-1,4-dithianium (1) was recently identified as the majorsolid constituent of ‘‘mustard heels’’ [1,2], a solid,gel-like substance arising from the decomposition ofmustard (bis (2-chloroethyl) sulfide) in storagecontainers. First proposed as an intermediate for thethermal decomposition of mustard [3],1 has beenknown for many years [4] and retains about 1/10 thetoxicity of mustard [5,6].1 and a related compound,S-(5-chloro-3-thiapentyl)-1,4-dithianium (2), were also

found in the aqueous phase during the thermal decom-position of mustard in the presence of water [7]. As1and 2 are potentially important as environmentalmarkers for mustard, their detection and identificationin various matrices such as soil is of current concern.

Initial attempts at characterizing an authenticsample of1 in its natural solid state by13C CP-MASNMR were complicated by broadened lines at roomtemperature. Such line-broadening in organic solids isoften caused by slow molecular motions [8,9]. There-fore, 1 and2 were investigated by variable tempera-ture 13C CP-MAS NMR to verify molecular motion asthe source of the line-broadening and to elucidate themotional mechanism. The motion of the side chains isreadily explained by the well-known ‘‘crankshaft’’

Journal of Molecular Structure 479 (1999) 93–101

MOLSTR 10689

0022-2860/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00810-2

* Corresponding author. Tel.:1 1 410 436 8468; Fax:1 1 410436 3764; e-mail: [email protected]

motion ofn-alkanes [10,11], but the motion involvingthe ring carbons is not so clear. Owing to the presenceof the side-chains, and resulting constraints imposedby the lattice, the motion cannot be because of rota-tion about the ring axis as in cyclohexane [12]. Thus, aconformational mechanism is implicated, thepresence of which is undoubtedly fostered by theapparent lack of conformational preference typicalof S-alkylthianium salts in solution [13]. Conforma-tional motion resulting in complete ring inversion haspreviously been proposed in the low-temperature,semi-rigid (non-plastic) phases of four-memberedrings such as cyclobutane [14] and thiacyclobutane[15]. But for six-membered rings, such conforma-tional motion and/or ring inversion has only beenobserved in highly-disordered systems, i.e. cyclo-hexane in the rather expansive cavities of inclusioncompounds [16] and perfluorocyclohexane in its high-temperature plastic phase [17]. As no precedentapparently exists for the conformational motion ofsix-membered rings in semi-rigid phases, a series ofmodel compounds (3–5, Scheme 1) were examined toverify the generality of the motion in cyclic sulfoniumions. Finally, compounds6–8 were also studied tocontrast the behavior of carbons in six-memberedrings undergoing hindered-rotation in semi-rigidphases.

2. Experimental

2.1. Materials

1 and3 were synthesized and isolated as the BF42

salt [18].2 was prepared by refluxing 1,4-dithiane and2-bromo-20-chloro-diethyl sulfide with AgBF4 innitromethane and isolated as the BF4

2 salt. Caution:2 slowly degrades at room temperature to formmustard [7]! 4 and 5 were prepared from methyl

iodide and 1,4-dithiane and thiane, respectively, andisolated as the I2 salt. The structures were character-ized by solution NMR as previously described [2].

2.2. NMR

Variable temperature13C CP-MAS NMR spectrawere obtained using a Varian UnityPlus 300 NMRspectrometer equipped with a Doty Scientific 7 mmVT-MAS probe. The decoupler field was about50 kHz and contact times of 0.5–2 ms were used.Liquid and solid samples were contained in doubleo-ring sealed macor rotors and spun at 3000 Hz.Liquid samples were spun at slower speeds (a fewhundred Hz) until frozen, before increasing to3000 Hz. The temperature range of the probe was11608C to about21208C. The Doty VT controllerand probe was calibrated with spinning (3000 Hz)samples of methanol (low temperature) and ethyleneglycol (high temperature) using the1H NMR chemicalshift difference method and spectrometer software.These measurements were within 58C of the tempera-ture indicated by the controller for the temperaturerange of 11008C to 21108C. Also, phase changeeffects observed for thiane at2 338C [22] and cyclo-hexane at2 878C [23] (given later) were also within58C of the indicated temperature, and theEa found forcyclohexane hindered-rotation was identical to thereported literature value [12]. Thus, a conservativeestimate of the temperature measurement error is^

58C. Spectra were obtained for6, 7 and8 using directexcitation (i.e., no CP) owing to concerns about extre-mely long protonT1s. For example, the trithianeprotonT1 exceeds 1000 s for suitably slow ring rota-tion [29]. Indeed, 13C CP-MAS spectra were notobtainable for 1,4-dithiane at room temperature,although direct excitation MAS spectra were. Spectrawere referenced to external TMS (0 ppm).

G.W. Wagner, Y.C. Yang / Journal of Molecular Structure 479 (1999) 93–10194

Scheme 1.

3. Results and discussion

3.1. Motional NMR line-broadening

For organic solids, NMR line-broadening (trans-verse relaxation timeT2) owing to a motion of corre-lation timet c is a function of the proton decoupling

powerv1 according to Eq. (1) [8,9].

T212 � 4g2

I g2s"2

� �= 15r6� �h i

× I I 1 1� ��

× tc= 1 1 v21t

2c

� �h i: �1�

As T2 is directly related to linewidth, two line

G.W. Wagner, Y.C. Yang / Journal of Molecular Structure 479 (1999) 93–101 95

Fig. 1. Solution13C NMR (top spectra) and13C CP-MAS NMR spectra of1 and2 at the following temperatures (top to bottom):1, left column,293, 253, 223, 173 K;2, right column, 293, 198, 173 K. Chemical shifts are given in Table 1.

narrowing regimes are expected atv1tc q 1 (slowexchange) andv1tc p 1 (fast exchange). Maximumline broadening occurs atv1tc � 1. Activation ener-gies (Ea) are easily determined by measuring the line-widths (D) at temperatures near the broadeningmaximum (Eq. (2)) [8,9].

D � D0exp 2Ea=RTÿ �

: �2�

3.2. Variable temperature13C CP-MAS NMR spectraof 1 and2

Selected variable temperature13C CP-MAS NMRspectra obtained for1 and2 are shown in Fig. 1, alongwith their solution spectra. Peak assignments aregiven in Table 1 using the numbering scheme shownin the structure of2 (see Scheme 1). At ambienttemperature, substantial broadening is observed inthe solid state spectrum, especially for the ringcarbons, resulting in overlapping and obscuring ofindividual peaks. As expected for motional

broadening, line narrowing is observed for1 attemperatures above and below the point of maximumbroadening (Tmax) as the rate of the motion involvingthe various carbons becomes fast and slow relative tothe decoupling field strength (ca. 50 kHz). This beha-vior is most apparent for the resolved ring carbons of1at 24 ppm. By 173 K, sufficient narrowing occurs topartially resolve the second ring carbon peak as ashoulder at 35.5 ppm. The motion for2 is not suffi-ciently slow at 173 K to observe narrowing of eitherthe ring or side-chain carbons. Compared to the ringcarbons in1 and2, the lines for the side chain carbonsare relatively sharp at ambient temperature, and attri-butable to the well-known fast ‘‘crankshaft’’ motionof alkane chains [10,11]. But greater broadening ofthe ring carbons indicates that these carbons areinvolved in a more-restricted motional process. Thefollowing is a discussion of the behavior of1, 2, andrelated ring systems3–8 (Fig. 2) delineating thedifferences between carbons involved in conforma-tional and rotational motions. Essential13C CP-MAS


Table 113C NMR dataa

Solution shiftsb Solid shiftsc Tmaxd Eae

1 43.1(7), 38.7(8), 37.7(2,6),23.6(3,5)

208C: 46.8, 37.2, 24.4 253 K 6.1

2 1008C: 45.9, 38.7, 35.5, 25.82 44.3(11),40.7(7), 37.3(2,6),

34.2(10),208C: 44.3, , 38.8, 27.4 , 173 K —

26.2(8), 23.6(3,5) 2 1008C: 35.8, 24.93 43.5(9),37.4(7), 37.3(2,6),

27.4(8),43.4(9),41.8(7), 37.5(2,6),26.9(8),

, 173 K —

23.4(3,5) 25.0(3,5)4 38.0(2,6), 23.0(3,5),22.4(7) 34.6,20.1 298 K 2.35 37.6(2,6), 23.0(4),22.4 (7),

21.0(3,5)208C: 38.4,27.0, 23.5 . 298 K —

2 1008C: 37.1,25.8, 23.4, 22.76 29.5(2,6), 28.4(3,5), 27.1(4)f 08C: 29.7, 28.7, 27.3g 218 K 3.8

2 1008C: 31.6, 28.1, 26.7h

7 29.1 958C: 30.4g; 208C: 30.2h 348 K 12.98 27.8f 2 258C: 28.1g; 2 1008C: 27.2h 188 K 11.4

a Spectra obtained at room temperature unless otherwise specified.C-7 carbon underlined.b In CD3CN. Ref. to internal CD3CN (1.3 ppm).c Some peaks not resolved owing to broadening. Ref. to external TMS (0 ppm).d Temperature of maximum broadening.e In kcal/mol.f Neat.g High temperature (plastic) phase.h Low temperature (ordered) phase.

and MAS NMR data is reported for these compoundsin Table 1.

3.3. Conformation and the C-7 shift

In solution,5 equilibrates into 40% axial and 60%equatorial conformations which are in fast exchangeat room temperature [19]. The shifts of the C-7carbons for the individual conformations are 19.0(axial) and 27.5 ppm (equatorial), yielding thereported average of 22.2 ppm [19] in agreementwith our solution value (Table 1). The high frequencyshift of the C-7 carbon in solid5 (27.0 ppm) indicatesadoption of the equatorial conformation at roomtemperature in the slow exchange regime (givenlater) consistent with the known X-ray structure[20]. For 1, a somewhat lesser high frequency shiftof the C-7 carbon is observed (46.8 vs. 43.1 ppm), butstill suggestive of a predominately equatorial confor-mation in the solid state near the slow exchangeregime. An X-ray structure determination couldconfirm this prediction [21]. Such a prediction ofconformation is not as straightforward for4 as all

carbons exhibit nearly identical low frequency shiftsin the solid compared to solution, yet the proximity ofthe C-7 shift (20.1 ppm) to that of the axial conforma-tion of 5 favors the axial conformer of4 predomi-nating in the solid near the slow-exchange regime.The axial conformer of4 is shown in Scheme 1.

Whereas the ring carbon peaks in the roomtemperature spectra of1, 4, and 5 are quite broad,the corresponding peaks are much narrower for2and3, as the motion for these carbons is in the fast-exchange regime (given later). As noted above for1and5, the high frequency C-7 shift of3 in the solid(41.8 vs. 37.4 ppm) suggests a predominantly equa-torial conformation for the ring, but in the fast-exchange regime. For2 the exact C-7 shift is obscuredby other overlapping peaks, but appears to be near thatof the solution value (40.7 vs. about 38.8 ppm). Thesevalues are quite close, suggesting a similar axial-equa-torial equilibrium constant for the solid. This would ofcourse require the conformational motion in2 to resultin complete ring inversion. However,Ea could not beobtained to support the suspected ring inversion(given later).


Fig. 2. Plots of variable temperature13C MAS NMR linewidths vs. inverse temperature. Phase changes for6 and8 are indicated by dashed lines.Solid lines denote linear regions, the slopes of which equalEa/R.

3.4. Ring motions in solids

Ring motions in solids are typically attributed torotations about the axis of the ring as in cyclohexane(8) [11,12], although an alternative mechanism invol-ving conformational motion and ring-inversion in thefour-membered rings of solid cyclobutane and thiacy-clobutane has been proposed [14,15]. Furtherevidence favoring the latter mechanism is the asym-metry of thiacyclobutane and the existence ofl -tran-sitions in heat capacity curves of both compounds.Both these traits are common to thiane (6) [22](given later). For six-membered rings in solids,conformational motion (including ring-inversion)has previously been reported for isolated cyclohexanemolecules in the rather expansive cavities of inclusioncompounds [16] and in the plastic phase of perfluor-ocyclohexane [17]. In high temperature ‘‘plastic’’phases, or near the melting point, in addition to rota-tion and inversion, rings can also undergo flips (1808rotations about in-plane axes) and even diffusion,yielding very narrow lines reminiscent of solutionspectra [11]. The extremely sharp linewidths(,10 Hz) found in13C MAS NMR spectra obtainedfor the high temperature solid phases of6 and8, andeven solid 7 at 958C (mp 1108C) reflect theseunrestricted isotropic motions. Further details of theresults for these rings are discussed later.

3.5. Ring motion mechanism for cyclic sulfonium ions

For cyclic sulfonium ions1–5, facile rotationsabout the ring axes are not possible owing to latticeconstraints imposed by the side chains. And although1808 flips about the long axes of the molecules arepossible, such unrestricted (plastic phase) motionshould cause more substantial line narrowing thanthat observed. For these compounds, only the

conformational mechanism is consistent with boththe broadening of the ring carbons and the similarsolution/solid state shifts of the conformationally-sensitive C-7 carbon for2 (given earlier). Thismechanism, illustrated in Scheme 2, shows thesuspected complete ring-inversion for2. Of course,lower amplitude conformational motions in the othercyclic sulfonium ion salts would fall short of ring-inversion. The mechanism does not require inversionat the sulfonium sulfur. Such an inversion is slow insolution, even at elevated temperatures [21]. Indeed,individual isomers, differing only in the sulfoniumsulfur configuration, may be isolated in their solidstate [21]. Thus, by coupling the side chain rotationwith inversion of the ring, the configuration of thenon-inverting sulfonium sulfur is preserved (the sulfo-nium sulfur merely rocks back and forth between thetwo conformers). An important detail of themechanism with regard to the suspected ring-inver-sion of2, is that both conformers occupy roughly thesame physical space and shape in the lattice. Variabletemperature X-ray structure determinations ofcompounds1–3 could confirm the room temperaturedisorder introduced by ring conformational motion, aswell as the adoption of the bent (equatorial?) structureshown in Scheme 2 at low temperature.

Consistent with this mechanism, the CP-MASNMR peaks for both pairs of ring carbons incompounds1–5 appear to broaden and narrow atthe same rate. In the low temperature spectra of1,maximum broadening of the ring carbons occurs ata much higher temperature than the side-chaincarbons, indicating that facile side-chain rotationcontinues after the ring conformational motionslows. Plots of13C CP-MAS NMR linewidths vs.inverse temperature for the ring carbons ofcompounds1 and4 are shown in Fig. 2 and activationenergies (Ea) associated with the conformationalmotions are reported in Table 1. As a result of peakoverlap, only one pair of ring carbons in eachcompound could be accurately measured, but theseinclude carbon pairs both next to the sulfonium sulfur(4) and the neutral sulfur (1), as indicated by asterisksin the structures shown in Scheme 1. For2 and3, thering motions are in the fast-exchange at roomtemperature as evidenced by broadening of the ringcarbon peaks at lower temperatures. ButTmax was toolow (, 21008C) for existing equipment, and their


Scheme 2.

Ea’s could not be obtained. For5, in the slow-exchange regime at room temperature, line-broadeningat elevated temperature was accompanied by decom-position, obviating itsTmax andEa determination.

3.6. Substituent effects on ring conformational motion

It appears that the conformational motion is facili-tated by longer side-chains as2 and 3, with chainlengths of at least three carbons, are in fast-exchangeat room temperature, but compounds1, 4 and5, withless than three carbons in the side-chains are in slow-exchange at room temperature. Facile motion ofthree-carbon side-chains is undoubtedly related tothe preference of three-bond motions in polyethylenechains at low temperature [10]. Indeed, the ability offacile chain motion to induce motion in the ring maybe the molecular equivalent of the ‘‘tail wagging thedog.’’ Besides three-carbon side-chains, conforma-tional motion also appears to be facilitated by thepresence of a second sulfur in the ring at the four-position asTmax for 4 is much lower than that of5.

3.7. Motions of cyclohexane, thiane and 1,4-dithianein non-plastic phases

Cyclohexane (8), which undergoes hindered-rota-tion in its low-temperature, non-plastic phase [12],and the increasinglyS-substituted rings, thiane (6)and dithiane (7), perhaps also undergoing similar rota-tions, were examined by13C MAS NMR to determinedistinguishing characteristics of carbons in rotationalvs. conformational motion. The variable temperature13C MAS NMR linewidths of these compounds areplotted in Fig. 2. For6 the total linewidth of thethree, merged peaks is plotted. Although this approx-imation involves an error owing to the chemical shiftrange of the peaks (2.4 ppm or 180 Hz, Table 1), thisadditional linewidth is constant and would haveminimal effect on the rate of line-broadening changewith temperature, i.e. the determination ofEa (givenlater). The temperatures of various phase changes for6 and 8 are indicated by dashed-lines. No phasechange data could be located for7, but the meltingpoint of 1108C is indicated.

The 13C MAS NMR spectra obtained for the non-plastic phases of6, 7 and8 show quite similar beha-vior to those of the cyclic sulfonium rings, i.e. thelinewidth broadens and narrows on either side of

Tmax. For cyclohexane, traversing from the plasticphase II to more-ordered phase I yields an abruptbroadening at 188 K, near the phase transition at186.09 K [23] as the isotropic motions cease andhindered-rotation begins. For 1,4-dithiane the onsetof broadening occurs near 363 K (evidentially theplastic-phase transition temperature) well below themelting point of 383 K, and similar hindered-rotationis implicated. Tmax is near 348 K. For thiane, thebroadening onset begins at about 228 K, intermediatebetween the two phase transitions of 240.02 and201.4 K [22]. In fact, first phase transition at240.02 K is actually accompanied by a slight peaksharpening prior to the broadening.Tmax is near218 K. Apparently, isotropic motions continue wellinto phase II, consistent with dielectric measurements[24], which are sensitive to reorientation of the mole-cular dipole moment. Significantly,Tmax is proximateto the onset of thel -transition at ca. 210 K in the heatcapacity curve [22], and analogous to the similarbehavior of cyclobutane [14] and thiacyclobutane[15], conformational motion is implicated. Dielectricmeasurements [24] of thiane in this vicinity suggestthat molecular rotation occurs in phase II, but rapidlyslows near thel-transition and ceases prior to phase I.As the observed13C MAS NMR line-broadening, andunderlying motion continues well into phase I, thesequence of events for thiane appear to be as follows:

1. 228 K, isotropic motions cease, rotational andconformational motion continues;

2. 210 K (l -transition, dielectric constant decreases),rotation slows, conformational motion continues;

3. 201 K (Phase transition II-I, dielectric constantnear rigid-molecule value), rotation ceases, confor-mational motion slows.

The low apparentEa for thiane betweenTmax at 218 Kand the 201.4 K phase transition suggest that theconformational motion does not involve completering inversion in this regime (given later).

3.8. Activation energies, conformational motion vs.rotation

Ea’s for the hindered ring rotations in cyclohexaneand 1,4-dithiane are 11.4 and 12.9 kcal/mol, respec-tively (Table 1), whereas theEa for the proposedcombined rotational/conformational motion for thiane


in the temperature range 218–198 K is 3.8 kcal/mol.TheEa’s for the conformational motions in the cyclicsulfonium ions1 and 4 are 6.1 and 2.3 kcal/mol,respectively. TheEa’s found for the conformationalmotions are much lower than those for hindered-rota-tion. However, available literature values indicate thatthe Ea’s for complete ring inversion in solution arevirtually identical to those of hindered ring rotationsin the solid state. Thus ring inversions in solids wouldbe difficult to distinguish from hindered rotation basedon Ea alone. The apparentEa for the conformationalmotion in solid thiane is remarkably close to ring-inversion barrier intimated by vapor heat capacities(DH(chair–boat)� 4.020 kcal/mol) [22], but bothbarriers are much lower than for inversion in solution(11.6 kcal/mol) [25], and the hindered-rotationascribed to the dielectric transition between 216–209.5 K (13.22 kcal/mol) [24]. For cyclohexane theEa for rotation in the solid (11.4 kcal/mol) is in excel-lent agreement with the rotation barrier determinedfrom wideline 1H NMR studies (11^ 1 kcal/mol)[12], and is also identical to theEa’s reported for itsconformational motion in the solid thiourea inclusioncompound (11.1 kcal/mol) [16] and in solution(11.24 kcal/mol) [26]. But the cyclohexane barrierDH(chair–boat) from heat capacity data is only5.6 kcal/mol [27]. TheEa apparent for hindered-rota-tion in dithiane (12.9 kcal/mol) is somewhat higherthan the value reported for inversion of 1,4-dithianein solution (10.3 kcal/mol) [28], but both values aresubstantially lower than that reported for ring rotationin solid 1,3,5-trithiane (21.4 kcal/mol) [29].

3.9. Extent of line-broadening, conformational vs.rotational motion

Empirically, ring carbons undergoing conforma-tional motion (including thiane) appear to have asmaller extent of line-broadening atTmax than theirrotating counterparts. This is quite evident in Fig. 2,where cyclohexane (8) and 1,4-dithiane (7) show thegreatest broadening (ca. 900–1300 Hz), whereassulfonium ions1 and4 and thiane (6) show less broad-ening (ca. 400–600 Hz). The extent of line-broad-ening is undoubtedly related to the underlyingmotional mechanism, i.e. hopping of the C–H vectorbetween two discrete positions (conformationalmotion) [14] and anisotropic rotation of the C–H

vector about a single axis, and apparently offers aqualitative means of discriminating the two motionalmechanisms.

4. Conclusions

The ambient temperature13C CP-MAS NMRspectra of solid cyclic sulfonium ions1 and2 sufferfrom motional broadening. Broadening in the sidechains is attributable to ‘‘crankshaft’’ motion typicalof alkane chains. Broadening of the ring carbons isconsistent with conformational motion, a generalmechanism for the series of cyclic sulfonium ionsstudied. Based on the apparent C-7 shifts and line-broadening behavior,1 adopts the equatorialconformer at ambient and sub-ambient temperature,whereas2 may be undergoing fast-exchange ring-inversion at room temperature. Maximum broadeningis observed at 253 K in1, but # 173 K in 2. Resolu-tion is greatly improved for1 at 173 K, permittingdetection of all carbon peaks. Much lower tempera-tures are needed to observe rigid-lattice spectra, espe-cially in the case of2.

Cyclohexane, 1,4-dithiane and thiane all exhibitisotropic motion in high temperature, plastic phases.Otherwise,13C MAS NMR detects little difference inthe behavior of the carbons in these rings in their lowtemperature phases and the carbons in the sulfoniumion rings. The results are consistent with hinderedrotations in the low temperature phases of cyclo-hexane and 1,4-dithiane. Thiane appears to exhibitboth conformational and rotational motion in its inter-mediate phase II, with rotation ceasing prior to thephase I transition at 201 K. Conformational motionappears to continue into phase I before the static spec-trum is observed. Conformational motion apparentlycauses less broadening than rotation in13C MASNMR spectra, a discriminating feature of the twomechanisms.

Acknowledgements

The authors thank Dr. George Hondrogiannis (NRCResearch Associate) and Dr. Louis P. Reiff (ERDEC)for the syntheses of the cyclic sulfonium ions. TheNIST Chemistry WebBook (http://webbook.nist.gov)


isalsoacknowledged for its databaseof thermochemicaldata, which facilitated locating phase change data.

References

[1] D.K. Rohrbaugh, Y.-C. Yang, J. Mass Spectrom. 32 (1997)1247–1252.

[2] Y.-C.Yang, L.L. Szafraniec, W.T. Beaudry, D.K. Rohrbaugh,Characterization of HD heels and the degradation of HD in toncontainers, in: Proceedings of the 1996 Scientific Conferenceon Chemical and Biological Defense Research, ERDEC-SP-048, (1997) pp. 353–360, APG, MD

[3] E.V. Bell, G.M. Bennett, A.L. Hock, J. Chem. Soc. (1927)1803–1809.

[4] M.A. Stahmann, J.S. Fruton, M. Bergmann, J. Org. Chem. 11(1946) 704–718.

[5] LD50� 75 mg/kg (mouse, IV) for 1 (4) compared to 8.5 mg/kg(mouse, IV) for mustard (6)

[6] W.P. Anslow, D.A. Karnofsky, B. Val Jager, H.W. Smith, J.Pharmacol. Exp. Therapeutics 48 (1948) 1–9.

[7] G.W. Wagner, B.K. MacIver, Y.-C. Yang, D.K. Rohrbaugh,Thermal degradation of mustard, in: Proceedings of the 1997Scientific Conference on Chemical and Biological DefenseResearch, APG, MD, in press

[8] W.P. Rothwell, J.S. Waugh, J. Chem. Phys. 74 (1981) 2721–2732.

[9] D. Suwelack, W.P. Rothwell, J.S. Waugh, J. Chem. Phys. 73(1980) 2559–2569.

[10] D. Hentschel, H. Silescu, H.W. Spiess, Macromolecules 14(1981) 1605–1607.

[11] E.R. Andrew, J. Chem. Phys. Chem. Solids 18 (1961) 9–16.

[12] E.R. Andrew, R.G. Eades, Proc. Roy. Soc. A216 (1953) 398–412.

[13] E.L. Eliel, R.L. Willer, J. Am. Chem. Soc. 99 (1977) 1936–1942.

[14] E.R. Andrew, J.R. Brookeman, J. Magn. Res. 2 (1970) 259–266.[15] D.F.R. Gilson, S. Katz, P.P. Saviotti, J. Magn. Res. 28 (1977)

243–246.[16] R. Poupko, E. Furman, K. Muller, Z. Luz, J. Phys. Chem. 95

(1991) 407–413.[17] J.D. Ellett Jr., U. Haeberlen, J.S. Waugh, J. Am. Chem. Soc.

92 (1970) 411–412.[18] G. Hondrogiannis, Y.-C. Yang, F.-L. Hsu, Synthesis and

structural identification of ω-chloroalkyl cyclic sulfo-nium salts, in: Proceedings of the 1996 Scientific Conferenceon Chemical and Biological Defense Research, ERDEC-SP-048, pp. 269–275, 1996, (1997) APG, MD

[19] G. Barbarella, P. Dembech, A. Garbesi, Tetrahedron 32(1976) 1045–1049.

[20] R. Gerdil, Helv. Chim. Acta 57 (1974) 489–493.[21] E.L. Eliel, R.L. Willer, A.T. McPhail, K.D. Onan, J. Am.

Chem. Soc. 96 (1974) 3021–3022.[22] J.P. McCullough, H.L. Finke, W.N. Hubbard, W.D. Good,

R.E. Pennington, J.F. Messerly, G. Waddington, J. Am.Chem. Soc. 76 (1954) 2661–2669.

[23] S. Kondo, M. Matsumoto, Bull. Chem. Soc. Jpn. 31 (1958)319–322.

[24] J.G. Aston, G.J. Szasz, H.L. Fink, J. Am. Chem. Soc. 65(1943) 1135–1139.

[25] J.B. Lambert, R.G. Keske, J. Org. Chem. 31 (1966) 3429–3431.

[26] F.A.L. Anet, A.J.R. Bourn, J. Am. Chem. Soc. 89 (1967) 760–768.

[27] C.W. Beckett, K.S. Pitzer, R. Pitzer, J. Am. Chem. Soc. 69(1947) 2488–2495.

[28] G. Hunter, R.F. Jameson, M. Shiralian, JCS Perkin II (1978)712–716.

[29] H.A. Resing, A.N. Garroway, Mol. Cryst. Liq. Cryst. 52(1979) 103–114.


13c cp-mas nmr studies of s-alkyl-1,4-dithianium salts: conformational motion in the solid state

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