Anal y sis of Pure Ro ta tional and Vi bra tion-rotational Spec tra of NaCl X 1 Σ + and Quan tum-chemical Cal cu la tion of Re lated Mo lec u lar Prop erties J. F. Ogilvie, a * H. J. Aa Jensen b and S. P. A. Sauer c a Escuela de Quimica, Universidad de Costa Rica, Ciudad Universitaria Rodrigo Facio, San Pedro de Mon tes de Oca, San Jose 2060, Costa Rica b Kemisk Institut, Syddansk Universitet, Campusvej 55, DK-5230 Odense M, Den mark c De part ment of Chem is try, Uni ver sity of Co pen ha gen, Universitetsparken 5, DK-2100 Co pen ha gen φ, Den mark We fit ted pub lished fre quency and wave num ber data for pure ro ta tional and vi bra tion-rotational spec tra, re spec tively, of 23 Na 35 Cl and 23 Na 37 Cl to de rive pa ram e ters re lated to po ten tial en ergy and to the ro ta tional g fac tor. For com par i son with these ex per i men tal data we un der took quan tum-chemical com pu ta tion of adi a - batic cor rec tions, ro ta tional and vi bra tional g fac tors, elec tric dipolar mo ment and its de riv a tive as a func tion of internuclear dis tance in a range near Re as a test of an al ge braic ap proach to spec tral anal y sis; ex per i men tal, 0.0287 ± 0.0014, and cal cu lated, 0.02149, val ues of gr at Re are in mod er ate agree ment. The com bined re sults are dis cussed from a point of view of com pu ta tional spec trom e try. Keywords: Spectral analysis; Vibration-rotational spectra; Quan tum-chemical cal cu la tions; Compu ta - tional spectrometry; Rotational and vi bra tional g factors. INTRODUCTION So dium chlo ride has played an im por tant role in the de- vel op ment of in fra red spec tros copy since the ear li est mea - sure ments in the mid in fra red re gion. For in stance, Ångstrom used a prism of min eral rocksalt to dis perse in fra red ra di a tion in his pi o neer ex per i ments, 1 and prac ti tio ners of in fra red spec trom e try since that time cus tom arily em ploy op ti cal com po nents of pure crys tal line NaCl such as win dows in cells to con tain gas eous and liq uid sam ples for spec tral ex am - i na tion. The rea son is that this com pound is in ex pen sive and trans mits light sat is fac to rily for wave lengths from the ul tra- vi o let re gion to 16 μm in the mid in fra red re gion. In the far- ther in fra red there is strong ab sorp tion of crys tal line NaCl with a max i mum about 32000 m -1 at trib uted to a lat tice vi - bra tion, called a trans verse op ti cal mode, in which sub- lattices of atomic cen tres of types Na and Cl os cil late with op po site phase. Whether these atomic cen tres cor re spond es - sen tially to atomic ions or to al most neu tral at oms is con tro - versial, 2 but in the gas eous phase at tem per a tures above 1100 K highly po lar di atomic mol e cules with stoichiometric for - mula NaCl are a ma jor con stit u ent of vapour over a sam ple of mol ten NaCl. Such a heated sam ple can emit ra di a tion re sult - ing from dis crete vi bra tion-rotational tran si tions from many states ther mally pop u lated un der those con di tions, 3 but spec- tra in ab sorp tion in mi cro wave, milli metre-wave and in fra red re gions are also readily ob serv able with ap pro pri ate in stru - ments. 4-7 For ro ta tional tran si tions, quan tum num ber J for ro - ta tional an gu lar mo men tum al ters by only one unit, where as for all ob served vi bra tion-rotational tran si tions vi bra tional quan tum num ber v con cur rently al ters by only one unit. 3,6 Re- ported spec tra in volve vi bra tional and ro ta tional states with max i mum val ues v = 8 and J = 118, re spec tively. In an anal y sis 8 of then avail able spec tra of mo lec u lar NaCl for com par i son of its pa ram e ters with those of other chlo rides of el e ments in group 1 - HCl, LiCl, KCl and RbCl – we as sumed a value for the ro ta tional g fac tor, g r , of 23 Na 35 Cl based on rea son able as sump tions ac cord ing to an anal y sis of this prop erty known for other mol e cules in elec tronic state 1 Σ + or 0 + . 9 To en sure that there be no sys tem atic de vi a tion be - tween mea sured wave num bers of tran si tions of 23 Na 35 Cl and 23 Na 37 Cl and those cal cu lated on the ba sis of pa ram e ters for po ten tial en ergy, 10 such in for ma tion is help ful or even es sen - tial. Be fore the pres ent work, no value of g r was known for NaCl from ei ther ex per i ment or re li able cal cu la tion of mo lec - u lar elec tronic struc ture. The ro ta tional g fac tor is a mul ti pli - Jour nal of the Chi nese Chem i cal So ci ety, 2005 , 52, 631-639 631 Ded i cated to Pro fes sor Ching-Erh Lin on the Oc ca sion of his 66 th Birth day and his Re tire ment from Na tional Tai wan Uni ver sity