5.76 Lecture #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)
5.76 Lecture #36S
Isomeric Forms
Page 2 of 10 pages
Figure 2-2.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
Table 2-2.
Table removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
5.76 Lecture #36S
Isomeric Forms
Page 3 of 10 pages
Figure 2-3.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
Table 2-3.
Table removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
5.76 Lecture #36S
Isomeric Forms
Page 4 of 10 pages
1
THE NON-PLANAR (NEAR-CIS) E A STATE OF ACETYLENE
I.
E − X , E − A , AND A − X , ALL ARE ELECTRIC DIPOLE ∴ E IS NON-CENTROSYMMETRIC
A.
E − X f ≈ 0.06 VERY STRONG
B.
A − X ZEEMAN POLARIZATION QUANTUM BEATS
C.
E − A QQ || VS. ⊥
II.
E − A : OBSERVE Ka = 1 ← Ka = 1 (a-TYPE)
SIGN OF ASYMMETRY SPLITTING IN E Ka = 1
⎧ TRANS − BENT 1 Bg
⎪
∴ IN PLANAR LIMIT ⎨
OR
⎪
1
⎩CIS − BENT A 2
III.
1
Bg ← X ∑ +g ( 1 Ag ) ALREADY RULED OUT
1
A2 ← X ∑ +g ( 1 A1 ) ELECTRONICALLY FORBIDDEN
1
1
ROTATIONAL SELECTION RULES FOR E ← A
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
IV.
NONPLANAR (NEAR-CIS) WITH TUNNELING THROUGH CIS BARRIER
E − X ν′′4 + ν′′5 HOT BANDS EXPLAINED
V.
INERTIAL DEFECT ⇒ NONPLANAR
5.76 Lecture #36S
Isomeric Forms
Page 5 of 10 pages
E -STATE IS NON-CENTRO-SYMMETRIC
A.
E ← X OBSERVED BY WILKINSON * IN ABSORPTION
TOO STRONG TO BE MAGNETIC DIPOLE: f < 10–3
B.
A ← X ZEEMAN POLARIZATION QUANTUM BEATS
B-FIELD
Z
Z
B-FIELD
ELECTRIC DIPOLE: ² MJ = ±1
MAGNETIC DIPOLE: ² MJ = 0
E-FIELD
X
⊥ POLARIZED
LIGHT
EXCITE WITH
SHORT PULSE
LASER
M+1
A
f ≈ 0.06
M
M–1
µm
µe
X M
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
A← i
X were electric dipole allowed.
Expected if i
*
P. G. Wilkinson, J. Mol. Spectrosc. 2, 387 (1958).
t
A← i
X were
Expected if i
magnetic dipole allowed.
5.76 Lecture #36S
Isomeric Forms
Page 6 of 10 pages
E ← A POLARIZATION DEPENDENT PQR INTENSITIES
C.
||
E
µ M ′M
⊥
2
0
–J
+J
–J
0
+J
A
A
Excitation sequence:
∴
|| STRONG
QQ ⊥ WEAK
–J
0
+J
X
NM
X
–J
0
+J
ELECTRIC DIPOLE SELECTION RULE: g↔u
E
u
g
A u
X g
E CAN BE NEITHER g NOR u because it
is excited from both rigorously g and
rigorously u initial states.
∴ E IS NOT CENTROSYMMETRIC
5.76 Lecture #36S
HCCH E
Isomeric Forms
Page 7 of 10 pages
v = 0 Ka = 1 ← A (0 0 2 0 0 0) Ka = 1
THE ONLY E − A BAND WITH
RESOLVABLE J-STRUCTURE
Figure 3-6.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
*
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
5.76 Lecture #36S
Isomeric Forms
Page 8 of 10 pages
PLANAR LIMIT
Ka = 1 LEVELS
TRANS
Ag
Au
Bg
a–
s+
s+
a–
CIS
Bu
A1
A2
211
212
B1
B2
s+
a–
a–
s+
X means NO
a–LEVELS
IN Ka = 1
A
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
FORBIDDEN FROM
OUT
X ∑ +g ( 1 A1 )
1
So it seems as though planar symmetry is ruled out!
5.76 Lecture #36S
Isomeric Forms
Page 9 of 10 pages
Figure 3-11.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
Figure 3-7.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
5.76 Lecture #36S
Isomeric Forms
Page 10 of 10 pages
Inertial Defect vs. A Rotational Constant
Figure 3-13.
Figure removed due to copyright restrictions. Please see: Lundberg, James K. "Double
Resonance Studies of Electronically Excited Acetylene.” MIT Chemistry, PhD Thesis,
February 1992.
1
E A
H
110°
75°
H
(ASSUMING SAME RCH AND RCC AS A -STATE)
A 1Au
H
120°
180°
H