2,5-Dithiacyclopentylideneketene (4). FVP of 1 at temperatures between 450 and 675 °C with Ar matrix isolation of the products at 12 K gave a ketene band at 2078, 2094 (shoulder) cm"1, increasing in intensity with the temperature. Bands due to C02 (2340 cm"1) and acetone were formed at the same time, and bands due to unreacted 1 decreased in intensity and had virtually disappeared at 700 °C.FVP of 1 at 675 °C (10-4 mbar) with neat isolation of the product at 77 K gave rise to a very strong ketene absorption at 2080 cm"1 together with bands due to acetone (Figure 2) [IR (4, 77 K) 2080 (vs), 1625, 1420, 1293, 1054, 917, 854 cm"1]. This material was stable on warming to -60 °C and disappeared on further warming to -10 °C. At -50 °C the half-life of the ketene was ca. 20 min.In an analogous experiment, 1 was pyrolyzed at 650 °C, and the product was collected at 77 K on a cold finger. C02 and the majority of acetone were pumped away at -60 °C, the cold finger was recooled, and CD2C12 was distilled onto the sample. Warming to -50 °C allowed the CD2C12 solution to flow into an NMR tube [ NMR (CD2C12, -45 °C) 3.70 (s)]. The IR spectrum of this solution confirmed the presence of the strong C=C=Q stretch at 2080 cm"1, surviving brief exposure to room temperature.The mass spectrum of 4 was obtained on both Kratos MS25RFA and Varían Mat 311 spectrometers with on-line FVP appliances. The temperature profiles of m/z 246 (1) and 144 (4) are shown in Figure 1. CAMS of m/z 144 (Variant Mat 311), m/z 116 (M+ -CO, 75%), 88 (41), 84 (36), 60 (100).Ethenedithione (5). Mass spectra of C2S2 (m/z 88) were obtained on the Kratos MS25RFA and Varían Mat 311 spectrometers with on-line FVP appliances. The temperature profile for C2S2 production from 1 is given in Figure 1 and from 2 and 3 in Figure 3. The CAMS of 5 (Varían Mat) produced by FVP of 1 at 590 °C, 2 at 778 °C, and 3 at 797 °C are shown in Figure 3. IR spectra (Ar, 12 K) were obtained on FVP of 1 at 700-1000 °C and of 2 and 3 at 600-1000 °C. Representative spectra are given in Figure 5: C2S2, 1180 cm'1; 34SCCS, 1176 cm (ca. 10%); S13CCS, 1163 cm"1 (ca. 2.5%). The UV spectrum similarly obtained by FVP of 3 at 700 °C is shown in Figure 6. Identical spectra of the 350-400-nm region were obtained by FVP of 1 (900 °C) and 2 (800 °C). Xmax 361, 363, 369, 370, 377, 378, 384, 385 (sh), 392 nm.The IR and UV bands ascribed to 5 disappeared on broad-band photolysis (1000 W high pressure Hg lamp) in 9 min. An increased but complex band at 1280 cm"1 developed during the irradiation and is ascribed to CS. Neat isolation of 5 at 12 K gave an IR band at 1170 cm"1, which disappeared on warming to 60 K. The use of an insert of quartz rods in the pyrolysis tube had hardly any effet on the strength of the IR signal observed for 5 at 1180 cm'1 as long as the pressure was 10"4 mbar.Use of a higher partial pressure created by fast sublimation of 1, 2, or 3 or pyrolysis temperatures above 1000 °C caused increased formation of CS and CS2 (1274 and 1527 cm'1, respectively) and partial or complete disap...