Molecular Spectra and Molecular Structure - Lec 36S

Isomeric Forms Page 1 of 10 pages Lecture #36 Supplement: C2H2 Has Many Isomeric Forms STRUCTURE MOLECULAR SYMMETRY SUBGROUP OF G8 a D!h Ca2 ( K a ) Cc2 ( K c ) E* (PARITY) [SYMMETRY OPERATION? PERMUTE H’s? PERMUTE C’s?] yes, no, no yes, no, no Cc2h no, no, no yes, yes, yes Cb2v no, no, no no, yes, yes Ca2v yes, yes, no no, ?, no Cb2 no, no, no no, yes, yes — Cc2 no, no, no yes, yes, yes — Rc(π) (–1)Kc Rc(π) Rc(π) (Near CIS) (Near Trans) a,b,c : CORRESPONDENCE BETWEEN Cn SYMMETRY AXIS AND a,b,c INERTIAL AXES CLASSIFY IN CNPI GROUP (G8) BECAUSE PERMUTATIONS AND INVERSIONS ARE RIGOROUS SYMETRY OPERATIONS. THE BEST (ONLY?) WAY TO DEAL WITH TRANSITIONS BETWEEN ISOMERIC FORMS (LARGE AMPLITUDE MOTIONS) Isomeric Forms Page 2 of 10 pages Isomeric Forms Page 3 of 10 pages Isomeric Forms Page 4 of 10 pages ! 1A STATE OF ACETYLENE THE NON-PLANAR (NEAR-CIS) E I. II. III. !, E ! , AND A !!X ! , ALL ARE ELECTRIC DIPOLE ∴ E ! IS NON-CENTRO !!X !!A E SYMMETRIC ! f ≈ 0.06 VERY STRONG !!X E A. !!X ! ZEEMAN POLARIZATION QUANTUM BEATS A B. ! QQ || VS. ⊥ !!A E C. ! : OBSERVE K = 1 ← K = 1 (a-TYPE) !!A E a a ! Ka = 1 SIGN OF ASYMMETRY SPLITTING IN E " TRANS ! BENT 1 Bg $ OR ∴ IN PLANAR LIMIT # $ 1 %CIS ! BENT A 2 1 ! 1 "+ ( 1 A ) ALREADY RULED OUT Bg ! X g g 1 ! 1 "+ ( 1 A ) ELECTRONICALLY FORBIDDEN A2 ! X g 1 ! !!A ROTATIONAL SELECTION RULES FOR E a-TYPE FROM Ka = 1 Ka NUMBERING CONFIRMED BY b-TYPE FROM Ka = 0, 2 WILKINSON’S A ! 8 cm–1 0 ! 1A2 BUT ELECTRONICALLY FORBIDDEN FRANCK-CONDON FORBIDDEN 0 00 BAND IV. NONPLANAR (NEAR-CIS) WITH TUNNELING THROUGH CIS BARRIER ! ! #"" + #"" HOT BANDS EXPLAINED !!X E 4 5 V. INERTIAL DEFECT ⇒ NONPLANAR Isomeric Forms Page 5 of 10 pages ! -STATE IS NON-CENTRO-SYMMETRIC E A. ! OBSERVED BY WILKINSON* IN ABSORPTION !!X E TOO STRONG TO BE MAGNETIC DIPOLE: f < 10–3 B. !!X ! ZEEMAN POLARIZATION QUANTUM BEATS A B-FIELD B-FIELD Z ELECTRIC DIPOLE: !MJ = ±1 MAGNETIC DIPOLE: !MJ = 0 Z E-FIELD X ! POLARIZED LIGHT EXCITE WITH SHORT PULSE LASER ! A f ≈ 0.06 M+1 M M–1 µm µe ! M X M INDUCED POLARIZATION ROTATES IN XY PLANE ! QUANTUM BEATS NO ROTATING POLARIZATION ! NO BEATS OBSERVED BY EVAN ABRAMSON AND PETER GREEN Ix Ix OR Iy Iy t t !!X ! were electric dipole allowed. Expected if A * P. G. Wilkinson, J. Mol. Spectrosc. 2, 387 (1958). t !!X ! were Expected if A magnetic dipole allowed. Isomeric Forms Page 6 of 10 pages ! POLARIZATION DEPENDENT PQR INTENSITIES !!A E C. || ! E µM !M " 2 0 –J +J 0 –J +J ! A ! A Excitation sequence: # QQ || STRONG QQ " WEAK –J 0 +J ! X NM ! X –J 0 +J ELECTRIC DIPOLE SELECTION RULE: g↔u ! E u g ! u A ! g X ! CAN BE NEITHER g NOR u because it E is excited from both rigorously g and rigorously u initial states. ! IS NOT CENTROSYMMETRIC !E Isomeric Forms ! HCCH E Page 7 of 10 pages v" = 0 Ka = 1 ! A (0 0 2 0 0 0) Ka = 1 ! BAND WITH !"A THE ONLY E RESOLVABLE J-STRUCTURE * MUST BE A Ka ≠ 0 LEVEL BECAUSE OF THE PRESENCE OF PQR BRANCHES. * INTENSITY RATIO PQR IMPLIES K′ a = 1 and a-type ROTATIONAL SELECTION RULES Isomeric Forms Page 8 of 10 pages PLANAR LIMIT TRANS Ag Au Ka = 1 LEVELS Bg a– s+ s+ a– CIS Bu A2 A1 211 212 B1 B2 s+ a– a– s+ X means NO a–LEVELS IN Ka = 1 ! 1A A u J=2 a+ s– 211 a+ ! a– 212 s– ! s+ IF TRANSITION TERMINATES IN Ka = 1, IT MUST BE a-TYPE. ∴ IT MUST GO TO LOWER ASYMMETRY COMPONENT (CONFIRMED BY TERM VALUE PLOT.) ! E IS 1 1 Bg OR A2 RULED ELECTRONICALLY OUT FORBIDDEN FROM ! 1 !+ ( 1 A ) X g So it seems as though planar symmetry is ruled out! 1 Isomeric Forms Page 9 of 10 pages b-type b-type b-type All nuclear permutation allowed levels must be either a+ or s–. a-type SELECTION RULE a+ → a– s– → s+ ONLY POSSIBLE FINAL STATES SPECIFIC EXAMPLES Isomeric Forms Inertial Defect vs. A Rotational Constant ! 1A E H 110° 75° H ! -STATE) (ASSUMING SAME RCH AND RCC AS A ! 1A A u H 120° 180° H Page 10 of 10 pages