An Indian Journal Trade Science Inc. Volume 2 Issue 4 OCAIJ, 2(4), 2006 [60-66] November 2006 Full P ull P ull P ull P ull Paper aper aper aper aper Tautomerism And Mass Spectra Of Thiomorpholides Eduardo A.Castro INIFTA (UNLP-CONICET-CIC), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Diagonal 113 y 64, Suc. 4, C. C. 16, (1900) La Plata, Buenos Aires, (ARGENTINA). E-Mail : castro@quimica.unlp.edu.ar, Tel. 54-221-4257430, Fax : 54-221-4254642. Patricia E.Allegretti, Facundo Namor, Jorge J.P.Furlong Laboratorio de Estudio de Compuestos Orgánicos (LADECOR), División Química Orgánica,Departamento de Química, Facultad de Ciencias Exactas,Universidad Nacional de La Plata, (1900) La Plata, Buenos Aires, (ARGENTINA). Received: 14 July, 2006 Accepted: 16 August, 2006  KEYWORDS Tautomerization; Thiomorpholides; Mass spectrometry; Theoretical calculations; AM1 Semiempirical Molecular Orbital Method. ABSTRACT The mass spectra of N,N-disubstituted thioamides generated by GC/ MS can provide valuable information about keto-enol equilibria occurring in gas phase. Mass spectra of selected thiomorpholides have been analysed and specific fragmentation assignments have been done to characterised and weigh co-existing keto and enol tautomers. The predictive value of this methodology is not only supported by the influence of substitution nature and size of these compounds on these equilibria but also by the good correlation found between the selected fragments abundances ratio and semiempirical calculation (AM1) of the corresponding heats of tautomerization. The results show that the thioketo-thioenol equilibrium can be studied by mass spectrometry where ionization in the ion source has a negligible effect on the position of that equilibrium and by GC/ MS where solvent separation in the analytical column seems to preclude shifts from the equilibrium reached at the injection port of the gas chromatograph.  2006 Trade Science Inc. -INDIA Web Publication Date : 14 th November, 2006 Organic CHEMISTRY Organic CHEMISTRY INTRODUCTION Thioamides are compounds of interest in sev- eral applicative areas, not only as intermediates of synthesis [1,2] but also in the field of the chemistry of peptides [3] .Among the members of this family of compounds, the thiomorpholides are particularly in- teresting since not only they exhibit diverse biologi-