Dihalogenated Methylperoxy Radicals: Spectroscopic Characterization and Photodecomposition by Release of HO^.

Abstract

Two atmospherically relevant dihalogenated methylperoxy radicals CHX2OO. (X=F and Cl) have been generated through O2‐oxidation of the corresponding alkyl radicals CHX2. in the gas phase. The IR spectroscopic characterization of both radicals in cryogenic Ar‐ and N2‐matrices (15 K) is supported by 18O‐labeling and ab initio calculations at the UCCSD(T)/aug‐cc‐pVTZ level. Upon 266 nm laser irradiation, both radicals decompose mainly by releasing hydroxyl radicals (→HO.+X2CO) via the intermediacy of intriguing α‐hydroperoxyalkyl radicals (.CX2OOH), implying that the photooxidation of dihalogenated hydrocarbons might serve as important sources of HO. radicals in the atmosphere.

Fluoromethylsulfinyl radicals: spectroscopic characterization and photoisomerization via intramolecular hydrogen shift

“…This assignment is supported by the good agreement with the B3LYP/6-311++G(3df,3pd) calculated IR frequencies at 1103/1125 and 1094/1088 cm À1 (Table 1) for the asymmetric (n asym ) and symmetric (n sym ) stretching modes of the CF 2 moiety in gauche and cis CHF 2 SO , respectively. The frequencies are lower than those for the same modes in CF 3 SO (n asym : 1195.6 and 1181.8 cm À1 ; n sym : 1142.8 cm À1 ), 15a CHF 2 OO (n asym : 1174.9 cm À1 ; n sym : 1139.0 cm À1 ), 19 and CF 3 OO (n asym : 1303.9 and 1261.8 cm À1 ; n sym : 1172.5 cm À1 ). 35 The existence of two conformers for CHF 2 SO is also evidenced by the observation of two bands at 1059.5 and 1056.3 cm À1 for the SQO stretching mode (n SO ), corresponding to the calculated frequencies at 1062 and 1065 cm À1 for the gauche and cis conformers of CHF 2 SO , and they are quite close to the same mode in other sulfinyl radicals such as CF 3 SO (1072.6 cm À1 ) 15a and CH 3 SO (1068.2 cm À1 ).…”

“…1A) with the concomitant appearance of strong IR bands for the known fragments (Fig. 1B) including CF 3 S˙ (a, 1147.0, 1144.4, 1124.6, and 760.6 cm −1 ), 23 CF 3 ˙ (c, 1247.7, 1083.7, and 701.0 cm −1 ), 33 ˙CHF 2 (e, 1311.9, 1186.3, and 1170.0 cm −1 ), 19 and SO 2 (1355.8, 1153.0, and 521.4 cm −1 ). 34 Additionally, two sets of IR bands at 1099.5/1104.4 and 1093.5/1091.7 cm −1 (Fig.…”

“…30 kcal mol −1 ), implying that the initially generated ˙CF 2 SOH and ˙CHFSOH might convert back to sulfinyl radicals under irradiation conditions (266 nm). Nevertheless, the S–O bonds in ˙CF 2 SOH (BDE = 5.2 kcal mol −1 ) and ˙CHFSOH (BDE = 10.4 kcal mol −1 ) are significantly stronger than the extremely weak O–O bonds in hydroperoxyalkyl radicals (˙QOOH) such as ˙CF 2 OOH (BDE = −62.8 kcal mol −1 ) 19 and ˙CH 2 OOH (BDE = −34.2 kcal mol −1 ), 18 which are highly unstable and undergo spontaneous dissociation by the release of ˙OH radicals in the photochemistry of the corresponding peroxy radicals CHF 2 OO˙ and CH 3 OO˙. Recently, a number of resonantly stabilized ˙QOOH radicals 43 have been characterized in the gas phase and cryogenic matrices.…”

“…Then, the vessel was connected to the vacuum line and all the volatiles in the reaction mixture were directed through three cold U-traps at À20, À65, and À196 1C. The desired product CHF 2 S(O)SCF 3 (38 mg, 0.2 mmol) was retained in the middle trap, and its quality was checked by NMR [Bruker Avance III HD 500 spectrometer, 1 H NMR: d = 6.6 (t) ppm; 19 F NMR: d = À112.8 (dd, -CHF 2 ), À115.2 (dd, -CHF 2 ), and À33.5 (s, -SCF 3 ) ppm] and gas-phase IR spectroscopy (INSA OPTICS FOLI10-R, n = 2974, 1337, 1276, 1184, 1130, 1112, 760, and 692 cm À1 ). By analogy, CH 2 FS(O)SCF 3 was prepared and its quality was also checked by NMR [ 1 H NMR: d = 5.6 (d) ppm; 19 F NMR: d = À205.2 (td, -CH 2 F) and À34.1 (s, -SCF 3 ) ppm] and gas-phase IR spectroscopy (n = 3004, 2946, 1310, 1179, 1147, 1112, 1075, and 762 cm À1 ).…”

“…18 For example, the prototypical molecules of ˙QOOH such as ˙CH 2 OOH, ˙CF 2 OOH, and ˙CCl 2 OOH decompose spontaneously by the release of ˙OH, and the attempts to generate these radicals through photoisomerization of the corresponding peroxy radicals at low temperatures were unsuccessful. 19 Structurally, CH 3 SO˙ can be viewed as a heavy congener of CH 3 OO˙. Theoretical calculations at the CCSD(T)/6-311G(d,p) level suggest that both ˙CH 2 SOH and CH 3 OS˙ are true minima on the potential energy surface of CH 3 SO˙, and the activation barrier for the rearrangement from CH 3 SO˙ to ˙CH 2 SOH is lower than that for the competing 1,2-CH 3 shift to CH 3 OS˙.…”