The gas phase Smiles rearrangement of anions PhO(CH(2))nO(-) (n = 2-4): A joint theoretical and experimental approach

A combination of experimental data [using 18O labelling fragmentation data together with metastable ion studies in a reverse sector mass spectrometer (from a previous study)] and ab initio reaction coordinate studies at the CCSD(T)/6-31++G(d,p)//B3LYP/6-31++G(d,p) level of theory, have provided the following data concerning the formation of PhO− in the gas-phase from energized systems PhO(CH2)nO− (n = 2–4). All ΔG values were calculated at 298 K. (1) PhO(CH2)2O− effects an ipso Smiles rearrangement (ΔGr = +35 kJ mol−1; barrier to transition state ΔG# = +40 kJ mol−1) equilibrating the two oxygen atoms. The Smiles intermediate reverts to PhO(CH2)2O− which then undergoes an SNi reaction to form PhO− and ethylene oxide (ΔGr = −24 kJ mol−1; ΔG# = +54 kJ mol−1). (2) The formation of PhO− from energized PhO(CH2)3O− is more complex. Some 85% of the PhO− formed originates via a Smiles intermediate (ΔGr = +52 kJ mol−1; ΔG# = +61 kJ mol−1). This species reconverts to PhO(CH2)3O− which then fragments to PhO− by two competing processes, namely, (a) an SNi process yielding PhO− and trimethylene oxide (ΔGr = −27 kJ mol−1; ΔG# = +69 kJ mol−1), and (b) a dissociation process giving PhO−, ethylene and formaldehyde (ΔGr = −65 kJ mol−1; ΔG# = +69 kJ mol−1). The other fifteen percent of PhO− is formed prior to formation of the Smiles intermediate, occurring directly by the SNi and dissociation processes outlined above. The operation of two fragmentation pathways is supported by the presence of a composite metastable ion peak. (3) Energized PhO(CH2)4O− fragments exclusively by an SNi process to form PhO− and tetrahydrofuran (ΔGr = −101 kJ mol−1; ΔG# = +53 kJ mol−1). The Smiles ipso cyclization (ΔGr = +64 kJ mol−1; ΔG# = +74 kJ mol−1) is not detected in this system.Tianfang Wang, Nico M. M. Nibbering and John H. Bowi

Diagnostic fragmentations of adducts formed between carbanions and carbon disulfide in the gas phase. A joint experimental and theoretical study

Selected carbanions react with carbon disulfide in a modified LCQ ion trap mass spectrometer to form adducts, which when collisionally activated, decompose by processes which in some cases identify the structures of the original carbanions. For example (i) C(6)H(5)(-) + CS(2)--> C(6)H(5)CS(2)(-)--> C(6)H(5)S(-) + CS, occurs through a 3-membered ring ipso transition state, and (ii) the reaction between C(6)H(5)CH(2)(-) and CS(2) gives an adduct which loses H(2)S, whereas the adduct(s) formed between o-CH(3)C(6)H(5)(-) and CS(2) loses H(2)S and CS. Finally, it is shown that decarboxylation of C(6)H(5)CH(2)CH(2)CO(2)(-) produces the beta-phenylethyl anion (PhCH(2)CH(2)(-)), and that this thermalized anion reacts with CS(2) to form C(6)H(5)CH(2)CH(2)CS(2)(-) which when energized fragments specifically by the process C(6)H(5)CH(2)CH(2)CS(2)(-)--> C(6)H(5)CH(2)(-)CHC(S)SH --> [(C(6)H(5)CH(2)CH[double bond, length as m-dash]C[double bond, length as m-dash]S) (-)SH] --> C(6)H(5)CH(2)CCS(-) + H(2)S. Experimental findings of processes (ii) and (iii) were aided by deuterium labelling studies, and all reaction profiles were studied by theoretical calculations at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory unless indicated to the contrary.Micheal J. Maclean, Scott Walker, Tianfang Wang, Peter C. H. Eichinger, Patrick J. Sherman and John H. Bowi

Contact Allergy to Dental Materials and Implants

Dental professionals and dental patients are exposed to the same sensitizers, but the outcome is very different. Dental professionals suffer predominantly from irritant contact dermatitis and to a lesser degree from allergic contact dermatitis to acrylates and rubber products. Dental patients rarely have allergic contact stomatitis. In this chapter, the reader will learn about the multiple factors that lead to irritant contact dermatitis and also about the major sensitizers, e.g., methacrylates in dentin bonding agents, dental composite resins (DCR), and prostheses, in addition to rubber chemicals, metals, fragrances, and disinfectants. The various sensitizers that may give lichenoid reactions in patients are also discussed