17452
J. Phys. Chem. 1996, 100, 17452-17459
Generation of New Nitrile N-Sulfides (NCCNS, R2NCNS, H3CSCNS, and ClCNS) as Ions and Neutrals in the Gas Phase: Tandem Mass Spectrometry, Flash Vacuum Pyrolysis, and ab Initio MO Study Robert Flammang,*,† Pascal Gerbaux,† Eva H. Mørkved,‡ Ming Wah Wong,*,§ and Curt Wentrup*,§ Organic Chemistry Laboratory, UniVersity of Mons-Hainaut, 19 AVenue Maistriau, B-7000 Mons, Belgium, Organic Chemistry Institute, UniVersity of Trondheim-NTH, N-7034 Trondheim, Norway, and Department of Chemistry, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed: June 25, 1996; In Final Form: August 30, 1996X
1,2,5-Thiadiazoles 1-5 bearing CN, CONH2, SCH3, and Cl substituents have been investigated under electron impact conditions. Collisional activation (CA) and neutralization-reionization (NR) mass spectrometries have allowed the identification of new nitrile N-sulfides (NCCNS, H2NCNS, H3CSCNS, and ClCNS) as ions or neutrals in the gas phase of a mass spectrometer. These results were supported by chemical ionization experiments of nitriles using carbon disulfide as the reagent gas, flash vacuum pyrolysis experiments, and ab initio molecular orbital calculations at the G2(MP2,SVP) level. The neutral nitrile N-sulfides are found by experiment and theory to be observable species in the gas phase. All the cations are potential energy minima, and their calculated fragmentation energies are in accord with experimental observations.
1. Introduction Nitrile N-sulfides, RCNS, are 1,3-dipoles of potential synthetic interest. However, in contrast to the well-known nitrile N-oxides, RCNO, the N-sulfides are short-lived intermediates that cannot usually be identified under ordinary reaction conditions.1 In addition to the fundamental importance of identifying short-lived molecules and establishing their involvement in chemical reactions, there is also interstellar chemical interest in these species.1,2 In this paper we wish to extend the knowledge of nitrile sulfides by characterization of several new members of the series using modern mass spectrometric methods such as neutralization-reionization mass spectrometry (NRMS),3 chemical ionization, and flash vacuum pyrolysis methods. Thiofulminic acid (HCNS) and some of its derivatives were identified unambiguously in previous work.1,2,4,5 In particular, benzonitrile N-sulfide was identified by a combination of flash vacuum pyrolysis (FVP), tandem mass spectrometry (MS/MS), and matrix isolation IR spectroscopy.2,4 Dinitrogen sulfide (N2S) is also a highly unstable molecule characterized only in the gas phase or in matrices.1,5 Most of the precursors of these reactive molecules were fivemembered heterocyclic compounds incorporating at least one CNS linkage. In this context, we decided to investigate more deeply the behavior upon electron impact ionization of some 1,2,5-thiadiazoles 1-5 recently synthesized by one of us.6 The
present investigation has allowed the identification of new nitrile †
University of Mons-Hainaut. University of Trondheim. § The University of Queensland. X Abstract published in AdVance ACS Abstracts, October 1, 1996. ‡
S0022-3654(96)01880-1 CCC: $12.00
N-sulfides in the mass spectrometer gas phase. Some of these experimental results are also supported by direct sulfuration of nitriles under chemical ionization conditions with carbon disulfide as the reagent gas, flash vacuum pyrolysis experiments, and quantum chemical calculations. High-level ab initio molecular orbital calculations play an important role in the description and characterization of these short-lived intermediates and their monocations and dications. Calculations provide valuable information on their electronic structures and fragmentation characteristics. 2. Experimental Section The electron ionization (EI), collisional activation (CA), and neutralization-reionization (NR) mass spectra were recorded on a large scale tandem mass spectrometer combining six sectors (VG AutoSpec6F, VG Analytical, Manchester, UK) of E1B1E2E3B2E4 geometry (E stands for electric sector and B for magnetic sector.7,8 The samples 1 and 4 were introduced in the ion source via a direct insertion probe, and samples 2, 3, and 5 were introduced via a heated (160 °C) septum inlet. General conditions were 70 eV ionizing electron energy, 200 µA trap current, 8 kV accelerating voltage, and 200 °C ion source temperature. In the CA experiments, a beam of ions is selected using the three first sectors at a mass resolution, eliminating any possible interference with isobaric ions, and subjected to collisional activation with O2 (80% transmittance (T)). In the MS/MS/ MS experiments, a beam of ions is mass-selected with E1B1 and collided with helium. The fragments of interest are subsequently selected by reducing the field of E2 and collided with oxygen. In the NR experiments, neutralization of the ions with methanol or ammonia (80% T) precedes reionization with O2 (also 80% T). Non-neutralized ions after neutralization were eliminated by floating the intermediate calibration ion source inserted between the two cells at 9 kV. All the spectra were recorded by scanning E3 and collecting the ions in the fifth fieldfree region. In the chemical ionization experiments, carbon disulfide (ca. 3-4 µL) was injected in the septum inlet, increasing the pressure © 1996 American Chemical Society
New Nitrile N-Sulfides of the source housing of the spectrometer to about 10-5 Torr. More experimental details will be reported elsewhere.9 The pyrolyzer used in the FVP/MS experiments has been described elsewhere.8 The thiadiazoles were prepared according to the literature (146 and 210). 3,4-Dichloro-1,2,5-thiadiazole 3 (Aldrich), cyanogen chloride 14 (Ucar), cyanamide 15 (Aldrich), methyl thiocyanate 16 (Aldrich), dimethylcyanamide 17 (Aldrich), pyrrolidine-N-cyanide 18 (Aldrich), and 2-amino-1,3,4-thiadiazole 10 (Aldrich) were commercially available. A sample of 2-amino-1,3,4,5-thiatriazole 11 was provided by Professor G. L’Abbe´, KUL, Belgium. Preparation of 3-Methylthio-1,2,5-thiadiazole (5). A solution of sodium sulfide nonahydrate (7.2 g, 30 mmol) in water (10 mL) was added to a solution of 3-chloro-1,2,5-thiadiazole 2 (2.4 g, 20 mmol) in 100% ethanol (10 mL). The solution, which turned bright red in 1 h, was stirred at ambient temperature for 1.5 h. The solvents were removed under reduced pressure at 40 °C, and the solid residue was extracted with 100% ethanol (2 × 20 mL). The ethanol was removed under reduced pressure, the residue was redissolved in 100% ethanol (50 mL), and some undissolved white solid was removed by filtration. The solution was concentrated almost to dryness, diethyl ether was added, and solid sodium 1,2,5-thiadiazole-3thiolate was filtered. Yield: 2.4 g (86%). Melting point: 260262 °C (dec). 1H NMR (DMSO): δ 7.81. IR (KBr): 32903240 (s, broad), 2148, 2074, 1649, 1437, 1292, 1262, 1142, 944, 864, 816, 791, 590 cm-1. Sodium 1,2,5-thiadiazole-3-thiolate (140 mg, 1 mmol) was dissolved in 100% ethanol (15 mL), iodomethane (0.7 g, 5 mmol) was added, and the solution was stirred at ambient temperature for 1 h. The solvent was removed under reduced pressure at 25 °C, and the residue was extracted with water (5 mL) and dichloromethane (10 mL). The dichloromethane extract was dried over magnesium sulfate, and the solvent was removed under reduced pressure to give a liquid residue, approximately 50 mg, with a characteristic odor. The compound, 3-methylthio-1,2,5-thiadiazole (5), is quite volatile. 1H NMR (CDCl3): δ 2.72 (3H, s), 8.29 (1H, s); signals for residual EtOH and CH2Cl2 were also present. 13C NMR (CDCl3): δ 14.55 (CH3), 147.32, 159.15 (C-SMe). 3. Results and Discussion a. Dissociative Ionization of 1,2,5-Thiadiazoles 1-5. Following electron impact, the molecular ions of 3,4-dicyano1,2,5-thiadiazole (1•+) (m/z 136, 80%) predominantly lose cyanogen, yielding m/z 84 ions (100%). Other peaks of minor importance in the mass spectrum of 1 are ascribed to CNS+ ions (m/z 58, 6%), NCCN•+ ions (m/z 52, 11%), and NS+ ions (m/z 46, 10%). The CA spectrum of the m/z 84 ions indicates the NCCNS connectivity as evidenced by the intense peaks at m/z 58 (loss of CN•), m/z 52 (loss of S), m/z 46 (NS+), and m/z 32 (S•+) (Figure 1a). In particular, the occurrence of an intense peak at m/z 46 for NS+ ions is not expected for isomeric ions like dicyanosulfide (NtCsSsCtN) or cyanoisothiocyanate (NtCsNdCdS) radical cations. The neutral counterpart of this last nitrile has been reported by Kroto et al.11 Another piece of evidence for the NCCNS connectivity is provided by an MS/MS/MS experiment on the m/z 58 ions observed in the CA spectrum of the m/z 84 ions. It is indeed possible to distinguish CNS+ ions from the CSN+ isomer by looking at their collision-induced dissociation.12 Collisional demethylation of CH3NdCdS•+ and CH3CtNfS•+ radical cations produces [C,N,S]+ cations, which differ by the relative
J. Phys. Chem., Vol. 100, No. 44, 1996 17453
Figure 1. (a) CA (O2) spectrum of the m/z 84 ions (NCCNS•+) generated by dissociative ionization of the thiadiazole 1 and (b) NR (NH3/O2) spectrum of the same ions.
SCHEME 1
abundances of their NS+ and CS•+ fragment ions produced by collision. The measured NS+/CS•+ ratio is 0.63 in the case of the isothiocyanate-like ions, NdCdS+, and 1.56 in the case of nitrile N-sulfide-like ions, CdNdS+. For 1, one measures 1.52, in good agreement with the production of CNS+ ions. That both CNS+ and NCS+ ions produce NS+ and CS•+ ions is at first glance surprising. We suspect that this peculiar behavior does not result from a postcollisional isomerization process.13 In fact, we have observed previously that “pure” SCN+ (NS+/CS•+ ratio reduced to 0.01) ions can be produced by charge reversal of the corresponding anions formed from methyl isothiocyanate in a chemical ionization source (self-CI conditions). The extent of isomerization seems therefore to depend on the method of production of the ions. The NR mass spectrum of the m/z 84 ions shown in Figure 1b is characterized by a very strong recovery signal corresponding to survivor ions and by fragmentations very similar to those observed in the CA spectrum. These combined pieces of information indicate that cyanogen N-sulfide, NCCNS (6), is a stable molecule when isolated in the mass spectrometer gas phase (Scheme 1). As usually observed, the charge-stripping peak in the CA spectrum is not detected in the NR spectrum. This has been rationalized by a lower probability for a m f m2+ reaction compared to a m+ f m2+ reaction.14 It is worthy to note that the oxygen analogue of 6, NCCNO, has been very recently produced and identified in the gas phase by a combination of experimental and theoretical techniques.15 The loss of HCN is the most important fragmentation pathway of the molecular ions of 3-chlorothiadiazole (2) (Scheme 2). The CA spectrum (Figure 2a) of the so-produced m/z 93 ions is again indicative of the nitrile N-sulfide connectivity: the base peak at m/z 58 corresponds to CNS+ ions (proved by an MS/ MS/MS experiment as described above). Note also that a
17454 J. Phys. Chem., Vol. 100, No. 44, 1996
Figure 2. (a) CA (O2) spectrum of the m/z 93 ions (ClCNS•+) generated by dissociative ionization of the thiadiazole 2 and (b) NR (NH3/O2) spectrum of the same ions. (The charge-stripping peak at m/z 46.5 is not seen at the resolution used here.)
Flammang et al.
Figure 3. (a) CA (O2) spectrum of the m/z 74 ions (H2NCNS•+) generated by dissociative ionization of the thiadiazole 4 and (b) NR (NH3/O2) of the same ions.
TABLE 1: CA(O2) Spectraa of [C,H2,N2,S]•+ Ions (m/z 74)
SCHEME 2
m/z precursor
58
47
46
42
37b
32
28
4 10 11 H2NCN + CI(CS2)
19 19 16 16
100 100 100 100
88 81 55 76
28 27 19 25
14 2
33 24 21 29
24 22 25 25
a
SCHEME 3
charge-stripping peak is again observed at m/z 46.5, indicating the stability of ClCNS2+ ions. The stability of cyanogen chloride N-sulfide (7) in the gas phase is also established by the NR spectrum (Figure 2b) where the structure characteristic peaks are again amplified compared to the CA spectrum [note, for example, the increased intensity of peaks at m/z 61 (loss of sulfur) and 47 (chlorocarbyne ions)]. As expected, the molecular ions of 3,4-dichlorothiadiazole (3) yield also abundant m/z 93 ions (loss of cyanogen chloride), and the CA and NR spectra were found to be identical with those obtained for the monochloro compound 2. Metastable molecular ions of 4-carboxamido-5-cyano-1,2,5thiadiazole (4) eliminate carbon monoxide (m/z 126). Such a decarbonylation reaction has already been reported for some primary amides like ionized propiolamide, a precursor of aminoethyne radical cations in the gas phase.16 We thus propose the formation of 4-amino-5-cyanothiadiazole radical cations, 8•+ (m/z 126) as shown in Scheme 3. A very small peak (about 1%) observed in the mass spectrum of 4 originates from these m/z 126 ions. The CA spectrum (Figure 3a) is characterized by intense peaks at m/z 58, 46, 42, 32, and 28, all suggesting
17
Main peaks indicated. b Charge-stripping peaks.
the cyanamide N-sulfide 9•+ structure. The base peak at m/z 47 is the sole spontaneous fragmentation of these ions; its formation may involve isomerization before dissociation. Calculations (vide infra) have indeed indicated that a low-lying isomer HNCNSH•+ exists on the CH2N2S•+ potential energy surface and that rearrangement of NH2CNS•+ to HNCNSH•+ has a moderate energy barrier. The intensity of this peak is drastically decreased in the NR mass spectrum (Figure 3b) probably because of a higher energy deposition in the NR experiment.17 The very high intensity of the recovery signal at m/z 74 is again indicative of the stability of the neutral sulfide H2NsCtNfS 7 in the gas phase. Dissociative ionization of 2-amino-1,3,4-thiadiazole (10) and 2-amino-1,3,4,5-thiatriazole (11) constitutes another source of CH2N2S radical cations (loss of HCN and N2, respectively). The CA spectra (Table 1) of these ions are very similar to the spectrum shown in Figure 3a. The same peaks are observed for the three ions with one noticeable difference: the disappearance of the charge-stripping peak in the case of 11•+. Moreover, this peak is barely seen in the spectrum of the m/z 74 ions from 10•+. We believe that ions 11•+ may be precursors of aminothiazirine radical cations 12•+ (Scheme 4); it is expected that such cyclic species will not readily accommodate the coexistence of two positive charges. The NR spectrum of ions 12•+ (Figure 4) is characterized by a strongly reduced intensity of the recovery signal (m/z 74) compared to 9•+ (Figure 3b). We suspect that the neutral aminothiazirine molecules do not survive the vertical neutralization step of the NR experiment and that the recovered ions m/z 74 are attributable to a minor contribution of cyanamide N-sulfide ions generated during the dissociative ionization of
New Nitrile N-Sulfides
J. Phys. Chem., Vol. 100, No. 44, 1996 17455 SCHEME 5
Figure 4. NR (NH3/O2) spectrum of the m/z 74 ions generated by dissociative ionization of the aminothiatriazole 11.
15%) implies that rearrangement of the molecular ions competes with the cycloreversion reaction. This is seen particularly in the case of the metastable molecular ions, which lose predominantly ammonia. It is therefore expected that the m/z 105 ions could in fact be a mixture of isomers. Evidence for this will be given below using chemical ionization with CS2 (CS3•+). b. Chemical Ionization Experiments with Carbon Disulfide.9 Electron ionization of carbon disulfide at a pressure of about 1 Torr gives mainly CS2•+ radical cations. However, a closer inspection of the mass spectrum indicates the existence of a significant amount of m/z 108 ions having the composition [C,S3]. The CA spectrum of these ions indicates the SCSS connectivity corresponding to the carbon disulfide S-sulfide structure reported by Su¨lzle et al. in the dissociative ionization of dithiole thiones.18 We have found that nitriles can be sulfurated by chemical ionization with CS2.9 By use of collisional activation, it has been unambiguously demonstrated that sulfur is transferred to the nitrogen atom, producing nitrile N-sulfide radical cations. In the context of the present work, these sulfur transfer reactions appeared quite interesting and were consequently applied to cyanogen chloride (14), cyanamide (15), and methyl thiocyanate (16) in order to produce the ionized sulfides proposed in the dissociative ionization of the thiadiazoles 2-5.
Figure 5. CA (O2) mass spectra of (a) m/z 105 (MeSCNS•+) and (b) m/z 90 ions (SCNS+) produced by dissociative ionization of 3-(methylthio)thiadiazole (5).
SCHEME 4
11. Then most of the ions at m/z 27-58 must be due to NR of neutral fragments (HCN, S, H2NCN, NS, and CNS). Similarly, dissociative ionization of 10 produces a mixture of structures with an increased proportion of ions 9•+ as indicated by the appearance of a charge-stripping peak in the CA spectrum (Table 1) and intensities of the peaks at m/z 46 and 32 between those recorded for 4 and 11. The mass spectrum of 3-(methylthio)thiadiazole (5) features intense peaks at m/z 132 (molecular ion, 100%), 105 (loss of HCN, 42%), and 90 (consecutive loss of HCN and a methyl group, 39%). The CA spectra of these fragment ions are shown in Figure 5. The strong loss of CH3• from the m/z 105 ions and the peaks at m/z 46 (NS+), 58 (CNS+), and 73 (CH3SCN•+) are indicative of the CH3SCNS connectivity of 13•+ (Scheme 5). This conclusion is reinforced by the CA spectrum of the m/z 90 ions, which is attributable to the cumulenic ion SdN+dCdS (Figure 5b). The presence in the mass spectrum of 5 of other fragment ions like m/z 99 (loss of SH•, 46%) or 115 (loss of ammonia,
Sulfuration of N,N-dimethylcyanamide (17) and N-cyanopyrrolidine (18) has also been attempted. The CA and NR mass spectra of cyanamide N-sulfide ions (6•+) and of cyanogen chloride N-sulfide ions (5•+) thus produced were found to be identical with the spectra shown in Figures 2 and 3 (see also Table 1). The corresponding results for dimethylcyanamide N-sulfide are shown in Figure 6. The CA spectrum of the m/z 102 ions features a very intense peak at m/z 46 (NS+), which is expected on the basis of the N-sulfide structure. The more intense loss of a methyl group is already observed without the collision gas and is presumably preceded by a shift of one hydrogen atom from the CH3 group prior to the simple cleavage. The NR spectrum is again characterized by a strong recovery signal and by intensification of peaks indicative of the connectivity, i.e., a reduced m/z 87 and an augmented m/z 58 (loss of C2H6N). It is, however, not fully understood why the latter fragmentation is weak in the CA spectrum. A similar behavior was noted for N-cyanopyrrolidine (18): formation of m/z 58 ions is only clearly indicated in the NR spectrum, not in the CA spectrum. Sulfuration of methyl thiocyanate 16 occurs readily under CI (CS2) conditions. The CA spectrum of the so-produced m/z 105 ions shown in Figure 7a is quite similar to the spectrum
17456 J. Phys. Chem., Vol. 100, No. 44, 1996
Figure 6. Sulfuration of dimethylcyanamide (15) by chemical ionization with CS2: (a) CA (O2) and (b) NR (NH3/O2) spectra of the m/z 102 ions (Me2NCNS•+).
Flammang et al.
Figure 8. (a) EI mass spectrum of 1 after flash vacuum pyrolysis and (b) CA (O2) spectrum of the m/z 168 ions (19 or 20).
SCHEME 6
Figure 7. Sulfuration of methyl thiocyanate (16) by chemical ionization with CS2: (a) CA (O2) and (b) NR (NH3/O2) spectra of the m/z 105 ions (MeSCNS•+).
depicted in Figure 5a. However, the disappearance of some peaks (e.g., the small m/z 88) arising from isomerization of the molecular ion of 5 before fragmentation is noteworthy. The NR spectrum is again characterized by a strong recovery signal (Figure 7b) and fragmentations, in keeping with the proposed connectivity. Methyl thiocyanate N-sulfide (13) is therefore a stable molecule when formed in the mass spectrometer gas phase. c. FVP/MS/MS of Thiadiazole 1. The complementarity of the NRMS and the FVP techniques has been stressed on many occasions. A major advantage of NRMS is the absence of wall collision, thus suppressing tautomerizations in particular.6 However, in contrast to FVP, NRMS has no preparative application. Short contact time FVP at 750 °C has been applied to the thiadiazole 1, real time analysis of the pyrolyzate being performed by tandem mass spectrometry. Thiadiazole 1 appears
to be very stable under these conditions, the molecular ion peak remaining intense. However, a significant decrease of the m/z intensity ratio 136/84 (0.8 at 200 °C and 0.6 at 750 °C) indicates that cyanogen N-sulfide 6 is also formed in the pyrolysis experiment. Owing to the high vapor pressure of 1, it is quite difficult to control accurately the rate of vaporization and, in fact, at higher pressure conditions, a new and very strong signal appears at m/z 168 (Figure 8a). These ions correspond formally to a dimer of cyanogen N-sulfide. Their CA spectrum (Figure 8b) presents a base peak at m/z 116 (loss of cyanogen); the consecutive reaction is the loss of CN•, giving [C,N,S2] cations (m/z 90). The observation of dimerization during FVP can also be considered as an argument for the formation of cyanogen N-sulfide. Such dimerization is not expected for isomers like NCsNdCdS or (NC)2S. Although of very low abundance (less than 0.5% of the base peak), ions corresponding to the sulfuration of the thiadiazole 1 are observed in the CI (CS2) mass spectrum. The CA spectrum of these m/z 168 ions is very similar to the spectrum shown in Figure 8b with some very minor differences in the relative abundances of the peaks, which can be ascribed to the different techniques of ionization used. This new information coupled with the fact that the m/z 168 ions consecutively eliminate NCCN and CN• supports the formation of a dicyanothiadiazole structure 19 bearing a sulfur atom on a ring nitrogen (Scheme 6). Since the CA spectrum of the CNS2•+ ions is identical with the spectrum shown in Figure 5b, one may expect that the loss of the CN radical is concerted with sulfur migration in order to generate the cumulenic ions SdN+dCdS. The present experiments cannot exclude the formation of the higher energy nitrile N-sulfide 20; the oxygen analogue of 19, the dimer of cyanogen N-oxide, has been reported recently.18
New Nitrile N-Sulfides
J. Phys. Chem., Vol. 100, No. 44, 1996 17457 TABLE 2: Calculated Relative Energies (kJ mol-1) of RCNS Neutralsa,b
Figure 9. Optimized (MP2/6-31G*) geometries of RCNS neutrals and radical cations (bond lengths in angstroms and angles in degrees).
d. Molecular Orbital Calculations. Computational Methods. Standard ab initio20 and density functional calculations were carried out with the GAUSSIAN 92/DFT series of programs.21 The structures and energies of RCNS neutrals, RCNS•+ radical cations, and related fragments were examined at the G2(MP2,SVP) level of theory.22 This corresponds effectively to QCISD(T)/6-311+G(3df,2p)//MP2/6-31G* energies together with zero-point vibrational and isogyric corrections. Spin-restricted calculations were used for closed-shell systems and spin-unrestricted ones for open-shell systems. The frozencore approximation was employed for all correlated calculations. Harmonic vibrational frequencies and infrared intensities were computed using the B3-LYP formulation23 of density functional theory, i.e., the Becke’s three-parameter exchange functional23a and the Lee-Yang-Parr correlation functional.23b The directly calculated B3-LYP/6-31G* frequencies were scaled by a factor of 0.9613 to account for the overestimation of calculated frequencies at this level of theory.24 The full set of optimized (MP2/6-31G*) equilibrium structures for NCCNS (6), ClCNS (7), NH2CNS (9), and CH3SCNS (13) and the corresponding radical cations are shown in Figure 9. The calculated relative energies of the RCNS neutrals and RCNS•+ radical cations are summarized in Tables 2 and 3, respectively. To facilitate future characterization of the RCNS cumulenes, we report also their rotational constants (MP2/631G*) and infrared spectra (B3-LYP/6-31G*) (Table 4). Structures and Stabilities of RCNS Neutrals. All nitrile N-sulfides (RCNS, R ) CN, Cl, NH2, and SCH3) are calculated to be stable with respect to all possible fragmentation processes (Table 2). In agreement with experimental findings, loss of a sulfur atom and loss of the R group are the most favorable dissociation pathways in all cases. For the parent compound (HCNS), CNSH is calculated to have similar energy, 2 kJ mol-1 above HCNS (G2(MP2,SVP)).25,26 We have also considered the structures and energies of CNSCN, CNSCl, and CNSNH2. As with the parent compound, the CNSR isomers are less stable than the corresponding nitrile N-sulfides (RCNS) by 34, 5, and 5 kJ mol-1, respectively, for R ) CN, Cl, and NH2. For NH2CNS (9), the isomeric carbodiimide structure HNdCdNsSH is predicted to be more stable than NH2CNS by 54 kJ mol-1. However, rearrangement of NH2CNS to ΗNCNSH, via a 1,4-H shift, is inhibited by an energy barrier of 160 kJ mol-1, and there is no experimental indication of such a rearrangement (see
speciesc
relative energy
NCCNS (6) CNSCN NCCN + S (S) CN• + CNS• CCN• + NS• N• + CCNS•
0.0 33.5 265.8 419.3 547.5 983.3
ClCNS (7) CNSCl ClCN + S (S) Cl• + CNS• CCl• + NS•
0.0 4.5 235.2 246.0 436.5
speciesc
relative energy
NH2CNS (9) HNCNSH CNSNH2 HNCNSH NH2CNS f HNCNSH TS NH2CN + S (S) H• + HNCNS• NH2• + CNS• CNH2• + NS•
0.0 -54.0 5.4 40.8 159.8 234.6 280.9 299.7 366.9
CH3SCNS (13) CH3• + SCNS• CH3SCN + S (S) CH3SC• + CNS• CH3SC• + NS•
0.0 197.2 256.1 264.3 438.2
a G2(MP2,SVP) E0 values. b Calculated G2(MP2,SVP) E0 energies include -583.089 03 (NCCNS), -950.109 90 (ClCNS), -546.249 35 (NH2CNS), and -927.951 35 (CH3SCNS) hartrees. c (S) denotes singlet state; TS denotes transition structure for the process indicated.
section c) in the neutral compound. Hence, all neutral nitrile N-sulfides considered here are predicted to be experimentally accessible and intrinsically stable species in the gas phase, in excellent agreement with experiment. All nitrile N-sulfides (6, 7, 9, and 13) are predicted to have a slightly bent CNS structure (∠CNS ) 172-178°, Figure 9). The calculated C-N bond lengths (1.186-1.208 Å) are somewhat longer than that of a typical CtN triple bond (e.g., 1.177 Å in HCN, MP2/6-31G*). Likewise, the N-S bond lengths (1.564-1.618 Å) are shorter than a N-S single bond (1.67 Å in S4N4H4),27 suggesting that the π electrons are delocalized in the CNS framework. The calculated N-S bond length in the nitrile N-sulfides is strongly influenced by the R substituent. The N-S bond length in NH2CNS (9) is rather long (1.618 Å), while a significantly shorter N-S bond length (1.564 Å) is calculated for NCCNS (6). Thus, NH2CNS is best described by the H2NsCtN+sS- resonance structure. To facilitate future experimental characterization of these cumulenes, calculated B3-LYP/6-31G* infrared spectra are reported (Table 4). It is interesting to note that there is a large variation of the cumulenic CNS stretching frequency. In accordance with the trend of the calculated CtN bond lengths in RCNS (Figure 9), a high CNS frequency (2290 cm-1) is calculated for NH2CNS (9) while a rather low CNS frequency of 2133 cm-1 is predicted for NCCNS (6). For acetonitrile sulfide (CH3CNS), the calculated B3-LYP/6-31G* (scaled) CNS frequency, 2230 cm-1, agrees well with the experimental value of 2252 cm-1.2 Structures and Stabilities of RCNS•+ Radical Cations. Consistent with the experimental findings, all nitrile N-sulfide radical cations (RCNS•+) are predicted to be stable with respect to all possible fragmentations (Table 3). When experimental fragmentation patterns with calculated energies of fragmentation are compared, it is important to realize that the CA spectra are composites of both collision-induced and unimolecular reactions, where unimolecular reactions in particular include rearrangement processes. Furthermore, the CA spectra represent a superposition of competitive and consecutive fragmentation reactions. Therefore, a perfect correlation between observed ion intensities and calculated fragmentation energies cannot be expected (a priori, the lower the threshold energy for the process, the more abundant the resulting ions may be). Nevertheless, as found in many cases,28 the agreement is good. The calculated fragmentation energies of NCCNS•+ (6•+) are in very good accord with the observed CAMS (Figure 1a). Formation of S•+ (m/z 32) is
17458 J. Phys. Chem., Vol. 100, No. 44, 1996
Flammang et al.
TABLE 3: Calculated Relative Energies (kJ mol-1) of RCNS Radical Cationsa,b speciesc
relative energy
speciesc
relative energy
NCCNS•+ (6•+) NCCN + S•+ (m/z 32) CCN• + NS+ (m/z 46) CN• + CNS+ (m/z 58) NS• + CCN+ (m/z 38) S (S) + NCCN•+ (m/z 52) CNS• + CN+ (m/z 26) N• + CCNS+ (m/z 70) CCNS• + N+ (S) (m/z 14)
0.0 351.6 406.1 506.4 591.7 731.0 769.4 834.6 1353.7
ClCNS•+ (7•+) CCl• + NS+ (m/z 46) NS• + CCl+ (m/z 47) ClCN + S•+ (m/z 32) Cl• + CNS+ (m/z 58) S (S) + ClCN•+ (m/z 61) CNS• + Cl+ (S) (m/z 35)
0.0 417.2 419.1 443.1 456.0 573.6 778.3
NH2CNS•+ (9•+) HNCNSH•+ HNCNSH•+ f NH2CNS•+ TS H• + HNCNS+ (m/z 73) NS• + CNH2+ (m/z 28) CNH2• + NS+ (m/z 46) S (S) + NH2CN•+ (m/z 42) NH2CN + S•+ (m/z 32) NH2• + CNS+ (m/z 58) CNS• + NH2+ (m/z 16) HNCNS• + H+ (m/z 1)
0.0 24.4 259.2 361.2 388.5 432.1 488.7 527.0 593.4 727.7 829.7
CH3SCNS•+ (13•+) CH3• + SCNS+ (m/z 90) SCNS• + CH3+ (m/z 15) CNS• + CH3S+ (S) (m/z 45) NS• + CH3SC+ (m/z 59) S (S) + CH3SCN•+ (m/z 73) CH3SC• + NS+ (m/z 46) CH3SCN + S•+ (m/z 32) CH3S + CNS+ (m/z 58)
0.0 283.8 354.2 434.9 445.7 450.5 482.3 527.4 536.8
a G2(MP2,SVP) E0 values. b Calculated G2(MP2,SVP) E0 energies include -582.719 37 (NCCNS•+), -949.786 72 (ClCNS•+), -545.958 37 (NH2CNS•+), and -927.652 32 (CH3SCNS•+) hartrees. c (S) denotes singlet state; TS denotes transition structure for the process indicated.
TABLE 4: Calculated Rotational Constantsa (GHz), Vibrational Frequenciesb (cm-1), and Infrared Intensitiesb,c (km mol-1) of RCNS NCCNS (6) A B C
3037.5026 1.4702 1.4695
ClCNS (7)
NH2CNS (9)
Rotational Constant 212.5689 242.4947 1.5728 2.4517 1.5612 2.4340
Vibrational Frequency and Intensity 2274 (558) 2210 (154) 3459 (48) 2133 (128) 918 (159) 3375 (31) 1083 (87) 484 (0) 2290 (27) 555 (23) 351 (14) 1596 (21) 457 (1) 343 (0) 1149 (165) 457 (1) 60 (14) 1133 (1) 392 (6) 578 (22) 392 (6) 481 (317) 115 (5) 369 (0) 115 (5) 354 (4) 196 (3) 74 (4)
CH3SCNS (13) 16.9736 1.1345 1.0707 3068 (2) 3054 (2) 2963 (15) 2142 (314) 1447 (15) 1433 (10) 1328 (8) 973 (14) 948 (4) 901 (81) 639 (2) 478 (7) 402 (9) 363 (1) 273 (5) 171 (0) 124 (0) 62 (0)
is likely that 9•+ can isomerize to HNCNSH•+ under the CAMS conditions. The carbodiimide structure would also explain the presence of the minor peaks at m/z 41 (HNCN+) and 59 (HSNC•+). Importantly, the m/z 47 signal is strongly reduced in the NR mass spectrum (Figure 3b). This supports the rearrangement hypothesis, since it is well-known that NR spectra are more characteristic of initial (i.e., unrearranged) ion structures16 (section a). An analogous rearrangement is, of course, not expected for N,N-dimethylcyanamide N-sulfide and also not observed (Figure 6). Here, a strong signal is observed at m/z 46 (NS+) but none at m/z 47 (cf. section b). The experimental spectrum of CH3SCNS•+ (13•+) (Figures 5a and 7a) is also in gross agreement with theoretical expectations. Note, however, that the abundant m/z 90 ions (SCNS+) themselves fragment to m/z 58, 46, and 32 (CNS+, NS+, and S•+; see Figure 5b). The signal at m/z 64 (S2•+) arises from both 13•+ and SNCS+ and is evidently due to isomerization. The ions NCCNS•+ (6•+), ClCNS•+ (7•+), and NH2CNS•+ •+ (9 ) are calculated to have linear R-C-N-S skeletons, while CH3SCNS•+ (13•+) is slightly bent (Figure 9). The calculated C-N and N-S bond lengths in RCNS•+ are similar to those of the corresponding neutrals (Figure 9). This is attributed to the fact that the highest occupied molecular orbital (HOMO) of RCNS is dominated by the atomic p orbital of the S atom.
a MP2/6-31G* optimized geometries. b B3-LYP/6-31G* values; frequencies scaled by 0.9613 (ref 24). c Intensity values are given in parentheses.
the most favorable fragmentation process. The very small peak at m/z 44 (CS•+) is evidently due to a rearrangement. All other peaks agree with predictions. For ClCNS•+ (7•+) too, the agreement with the observed spectrum (Figure 2) is very good. Only the small signal at m/z 67 (ClS+) must be due to rearrangement. The same spectrum is obtained by sulfuration of ClCN (section b). For NH2CNS•+ (9•+), agreement with experiment (Figure 3) is good except for the strong signal at m/z 47 (HNS•+), which cannot be formed from unrearranged ion 9•+. The carbodiimide isomer HNdCdNsSH•+ is calculated to be a stable equilibrium structure with energy close to NH2CNS•+ (24 kJ mol-1 above 9•+). Rearrangement of 9•+ to HNCNSH•+, via a 1,4-H shift, has a calculated barrier of 259 kJ mol-1, well below the threshold energy for the easiest fragmentation of 9•+. Thus, it
The geometrical similarity is in accord with the observation of strong “recovery signals” in all the NR spectra. For instance, vertical neutralization of ground-state NH2CNS•+ (6) should lead to neutral NH2CNS (6) molecules with just 8 kJ mol-1 internal energy. RCNS2+ Doubly Charged Cations. For all nitrile N-sulfide radical cations (6•+, 7•+, 9•+, and 13•+), charge-stripping is clearly observed in the CA(O2) spectra. Efficient chargestripping processes during collisional activation are commonly found in cumulenes.14b,17c,28,29 Consistent with the experimental findings, NCCNS2+, ClCNS2+, NH2CNS2+, and CH3SCNS2+
New Nitrile N-Sulfides dications are calculated to be thermodynamically stable species in the gas phase. The high stability of these doubly charged ions may be partly attributed to the fact that an S atom is able to accommodate positive charge better than first-row atoms. In contrast to the corresponding neutrals (Figure 9), the RCNS2+ dications have significantly longer C-N (1.21-1.24 Å) bond lengths and shorter N-S bond lengths (1.51-1.56 Å). This again may be understood by considering the effect of double ionization of the RCNS neutrals. On going from the RCNS neutral to the RCNS2+ dication, two electrons are removed from the HOMO, which is C-N bonding and N-S antibonding. As a consequence, there is a lengthening of the C-N bond but a shortening of the N-S bond. Such structural changes are also observed for the RCNS•+ monocations (Figure 9) but with a smaller magnitude. 4. Conclusions Nitrile N-sulfide radical cations, NCCNS•+, H2NCNS•+, H3CSCNS•+, and ClCNS•+), have been generated by dissociative ionization and characterized by tandem mass spectrometry techniques. The stabilities of the corresponding neutrals have been investigated by neutralization-reionization, chemical ionization, and flash vacuum pyrolysis experiments, as well as ab initio MO calculations. The neutrals NCCNS, H2NCNS, H3CSCNS, and ClCNS are predicted to be observable species in the gas phase, in excellent agreement with the experimental results. All the cations are predicted to be stable species. Good agreement is observed between calculated and experimentally observed stabilities and fragmentation patterns. Acknowledgment. The Mons laboratory thanks the Fonds National de la Recherche Scientifique for its contribution toward the acquisition of a large-scale tandem mass spectrometer, the VG AutoSpec 6F. The Brisbane laboratory thanks the Australian Research Council for financial support and for a research fellowship for M. W. Wong. The authors also thank Professor G. L’Abbe´ for a gift of 2-amino-1,3,4,5-thiatriazole. References and Notes (1) Wentrup, C.; Kambouris, P. Chem. ReV. 1991, 91, 363. (2) Kambouris, P.; Plisnier, M.; Flammang, R.; Terlouw, J. K.; Wentrup, C. Tetrahedron Lett. 1991, 32, 1487. (3) For reviews, see the following. (a) Wesdemiotis, C.; McLafferty, F. W. Chem. ReV. 1987, 87, 485. (b) Terlouw, J. K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1987, 26, 805. (c) Holmes, J. L. Mass Spectrom. ReV. 1989, 8, 513. (d) McLafferty, F. W. Science 1990, 247, 925. (e) Plisnier, M.; Flammang, R. Chim. NouV. 1990, 8, 893. (f) Goldberg, N.; Schwarz, H. Acc. Chem. Res. 1994, 27, 347. (4) Maquestiau, A.; Flammang, R.; Plisnier, M.; Wentrup, C.; Kambouris, P.; Paton, M. R.; Terlouw, J. K. Int. J. Mass Spectrom. Ion Processes 1990, 100, 477. (5) Wentrup, C.; Fischer, S.; Maquestiau, A.; Flammang, R. J. Org. Chem. 1986, 51, 1908. (6) Mørkved, E. H.; Kjøsen, H.; Neset, S. M. Acta Chem. Scand. 1994, 48, 372.
J. Phys. Chem., Vol. 100, No. 44, 1996 17459 (7) Bateman, R. H.; Brown, J.; Lefevere, M.; Flammang, R.; Van Haverbeke, Y. Int. J. Mass Spectrom. Ion Processes 1992, 115, 205. (8) Brown, J.; Flammang, R.; Govaert, Y.; Plisnier, M.; Wentrup, C.; Van Haverbeke, Y. Rapid Commun. Mass Spectrom. 1992, 6, 249. (9) Gerbaux P.; Flammang, R. In preparation. (10) Weinstock, L. M.; Davis, P. Handelsman, B.; Tull, R. J. Org. Chem. 1967, 32, 2823. (11) King, M. A.; Kroto, H. W.; Landsberg, B. M. J. Mol. Spectrosc. 1985, 113, 1. (12) Plisnier, M. Ph.D. Thesis, University of Mons-Hainaut, Mons, Belgium, 1992. (13) Nguyen, M. T.; Allaf, A. W.; Flammang, R.; Van Haverbeke, Y. J. Mol. Struct.: THEOCHEM, submitted. (14) (a) Wiedman, F. A.; Cai, J.; Wesdemiotis, C. Rapid Commun. Mass Spectrom. 1994, 8, 804. (b) Flammang, R.; Van Haverbeke, Y.; Wong, M. W.; Wentrup, C. Rapid Commun. Mass Spectrom. 1995, 9, 203. (15) Pazinski, T.; Westwood, N. P. C. J. Chem. Soc., Chem. Commun. 1995, 1901. (16) (a) Van Baar, B.; Koch, W.; Lebrilla, C.; Terlouw, J. K.; Weiske, T.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 827. (b) Buschek, J.; Holmes, J. L.; Lossing, F. P. Org. Mass Spectrom. 1986, 21, 729. (17) (a) Harnis, D.; Holmes, J. L. Org. Mass Spectrom. 1994, 29, 213. (b) Kuhns, D. W.; Tran, T. B.; Shaffer, S. A.; Turecek, F. J. Phys. Chem. 1994, 98, 4845. (c) Flammang, R.; Landu, D.; Laurent, S.; BarbieuxFlammang, M.; Kappe, C. O.; Wong, M. W.; Wentrup, C. J. Am. Chem. Soc. 1994, 116, 2005. (d) Flammang, R.; Laurent, S.; Flammang-Barbieux, M.; Wentrup, C. Org. Mass Spectrom. 1993, 28, 1161. (18) Su¨lzle, D.; Egsgaard, H.; Carlsen, L.; Schwarz, H. J. Am. Chem. Soc. 1990, 112, 3750. (19) Paszinski, T.; Ferguson, B.; Westwood, N. P. C. J. Chem. Soc., Perkin Trans 2 1996, 179. (20) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92/DFT; Gaussian Inc.: Pittsburgh, PA, 1992. (22) (a) Smith, B. J.; Radom, L. J. Phys. Chem. 1995, 99, 6468. (b) Curtiss, L. A.; Redfern, P. C.; Smith, B. J.; Radom, L. J. Chem. Phys. 1996, 104, 5148. (23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (24) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391. (25) Previous Hartree-Fock calculations predict CNSH to be more stable than HCNS by 55 kJ mol-1. Bak, B.; Christiansen, J. J.; Nielsen, O. J.; Svanholt, H. Acta Chem. Scand., Ser. A 1977, 31, 666. (26) HSCN and HNCS are predicted to lie 122 and 155 kJ mol-1 (G2(MP2,SVP)) below HCNS. However, there is no evidence for isomerization of the RCNS species to the lower energy isomers (RSCN and RNCS) in this work. Such isomerizations are stepwise processes through the metastable CNSR isomers. (27) Sass, R. L.; Donohue, J. Acta Crystallogr. 1958, 11, 497. (28) Wong, M. W.; Wentrup, C.; Mørkved, E. H.; Flammang, R. J. Phys. Chem. 1996, 100, 10536, and references therein. (29) (a) Wong, M. W.; Wentrup, C.; Flammang, R. J. Phys. Chem. 1995, 99, 16836. (b) Wong, M. W. J. Mass Spectrom. 1995, 30, 144. (c) Flammang, R.; Van Haverbeke, Y.; Laurent, S.; Barbieux-Flammang, M.; Wong, M. W.; Wentrup, C. J. Phys. Chem. 1994, 98, 5801. (d) Flammang, R.; Van Haverbeke, Y.; Wong, M. W.; Ru¨hmann, A.; Wentrup, C. J. Phys. Chem. 1994, 98, 5801. (e) Koch, W.; Maquin, F.; Stahl, D.; Schwarz, H. Chimica 1985, 39, 376. (f) Rabrenovic, M. R.; Proctor, C. J.; Ast, T.; Herbert, C. G.; Brenton, A. G.; Beynon, J. H. J. Phys. Chem. 1983, 87, 3305. (g) Ast, T. AdV. Mass Spectrom. 1980, 8A, 555.
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