1. Methylindoles – Microwave Spectroscopy
1-7 methyl substituted Indoles
What can we learn about the electronic environment of the methylindoles using internal rotation and quadrupole coupling?
Continuation of previous work, presented at ISMS 2014 – RJ03 MW spectroscopy of 1-,2-,3- and 5-methylindoles
RANIL M. GURUSINGHE, MICHAEL TUBERGEN
Department of Chemistry, Kent State University, Ohio

2. Methods Methylindoles – Microwave Spectroscopic Study
Recorded and assigned the rotational spectra of 1-7 methylindoles
Determined 14N quadrupole coupling constants (χaa, χbb, χcc)
Determined the potential barriers to methyl internal-rotation (V3)
Computational calculations at MP2 and DFT level:
rotational constants
potential barriers
Compared the χaa, χbb, χcc, and V3
Fabry–Pérot cavity based FT-Microwave Spectrometer
at Kent State University
7.5” diameter mirrors – 21 GHz
2.4 kHz resolution
Doppler splitting . 13 kHz FWHM of each component
Reservoir nozzle . Can be heated up to 200 °C
Methylindoles – Microwave Spectroscopic Study

3. Methylindoles – Microwave Spectroscopic Study
MW Spectra
First MW spectroscopic investigation on 1-7 methylindoles
methyl internal rotation splitting
14N quadrupole splitting
Methylindoles – Microwave Spectroscopic Study

4. Methylindoles – Microwave Spectroscopic Study
V3 Barrier
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHz
(15)
(13)
(4)
(6)
(5)
(9)
(13)
B/MHz
(7)
(3)
(2)
(4)
(12)
(2)
(11)
C/MHz
(4)
(2)
(2)
(9)
(5)
(9)
(7)
Pcc/u Å2
1.690
1.666
1.633
1.629
1.656
1.638
ΔJ/kHz
0.041(4)
0.0105(15)
0.048(2)
0.0409(11)
0.0145(5)
0.0035(7)
0.043(3)
ΔJK/kHz
-0.06(2)
0.02(2)
-0.072(7)
fixed 0.00
0.030(6)
-0.020(8)
δJ/kHz
0.013(2)
0.0012(11)
0.0156(7)
0.0158(7)
0.0037(2)
0.0172(9)
δK/kHz
0.11(3)
0.117(11)
0.053(6)
0.044(13)
-0.34(3)
0.042(9)
Dpi2J/MHz
0.048(4)
0.17(6)
0.056(5)
(2)
(4)
0.17(4)
Dpi2K/MHz
-0.15(7)
-0.5(2)
-0.097(12)
0.399(4)
0.397(6)
-0.29(6)
Dpi2-/MHz
0.021(5)
0.040(5)
(2)
(5)
0.16(4)
χaa/MHz
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
V3/cm-1
277.1(2)
374.32(4)
414.(5)
331.6(2)
(15)
(4)
426(3)
∠(i,a)/º
50.39(13)
54.(2)
65.29(7)
16.671(6)
18.118(9)
71.3(6)
∠(i,b)/º
39.61(13)
fixed 90.00
35.(2)
24.71(7)
73.328(6)
71.882(9)
18.7(6)
No. A lines 80 79 86 95 63 83
No. E lines 72 74 75 88 57 87
Δνrms/kHz
7.2
2.8
1.9
1.4
2.1
2.3
a-including the hyperfine components
Methylindoles – Microwave Spectroscopic Study

5. Quality of the XIAM Fits
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHz
(15)
(13)
(4)
(6)
(5)
(9)
(13)
B/MHz
(7)
(3)
(2)
(4)
(12)
(2)
(11)
C/MHz
(4)
(2)
(2)
(9)
(5)
(9)
(7)
Pcc/u Å2
1.690
1.666
1.633
1.629
1.656
1.638
ΔJ/kHz
0.041(4)
0.0105(15)
0.048(2)
0.0409(11)
0.0145(5)
0.0035(7)
0.043(3)
ΔJK/kHz
-0.06(2)
0.02(2)
-0.072(7)
fixed 0.00
0.030(6)
-0.020(8)
δJ/kHz
0.013(2)
0.0012(11)
0.0156(7)
0.0158(7)
0.0037(2)
0.0172(9)
δK/kHz
0.11(3)
0.117(11)
0.053(6)
0.044(13)
-0.34(3)
0.042(9)
Dpi2J/MHz
0.048(4)
0.17(6)
0.056(5)
(2)
(4)
0.17(4)
Dpi2K/MHz
-0.15(7)
-0.5(2)
-0.097(12)
0.399(4)
0.397(6)
-0.29(6)
Dpi2-/MHz
0.021(5)
0.040(5)
(2)
(5)
0.16(4)
χaa/MHz
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
V3/cm-1
277.1(2)
374.32(4)
414.(5)
331.6(2)
(15)
(4)
426(3)
∠(i,a)/º
50.39(13)
54.(2)
65.29(7)
16.671(6)
18.118(9)
71.3(6)
∠(i,b)/º
39.61(13)
fixed 90.00
35.(2)
24.71(7)
73.328(6)
71.882(9)
18.7(6)
No. A lines 80 79 86 95 63 83
No. E lines 72 74 75 88 57 87
Δνrms/kHz
7.2
2.8
1.9
1.4
2.1
2.3
a-including the hyperfine components
Methylindoles – Microwave Spectroscopic Study

6. Quality of the XIAM Fits
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHz
(15)
(13)
(4)
(6)
(5)
(9)
(13)
B/MHz
(7)
(3)
(2)
(4)
(12)
(2)
(11)
C/MHz
(4)
(2)
(2)
(9)
(5)
(9)
(7)
Pcc/u Å2
1.690
1.666
1.633
1.629
1.656
1.638
ΔJ/kHz
0.041(4)
0.0105(15)
0.048(2)
0.0409(11)
0.0145(5)
0.0035(7)
0.043(3)
ΔJK/kHz
-0.06(2)
0.02(2)
-0.072(7)
fixed 0.00
0.030(6)
-0.020(8)
δJ/kHz
0.013(2)
0.0012(11)
0.0156(7)
0.0158(7)
0.0037(2)
0.0172(9)
δK/kHz
0.11(3)
0.117(11)
0.053(6)
0.044(13)
-0.34(3)
0.042(9)
Dpi2J/MHz
0.048(4)
0.17(6)
0.056(5)
(2)
(4)
0.17(4)
Dpi2K/MHz
-0.15(7)
-0.5(2)
-0.097(12)
0.399(4)
0.397(6)
-0.29(6)
Dpi2-/MHz
0.021(5)
0.040(5)
(2)
(5)
0.16(4)
χaa/MHz
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
V3/cm-1
277.1(2)
374.32(4)
414.(5)
331.6(2)
(15)
(4)
426(3)
∠(i,a)/º
50.39(13)
54.(2)
65.29(7)
16.671(6)
18.118(9)
71.3(6)
∠(i,b)/º
39.61(13)
fixed 90.00
35.(2)
24.71(7)
73.328(6)
71.882(9)
18.7(6)
No. A lines 80 79 86 95 63 83
No. E lines 72 74 75 88 57 87
Δνrms/kHz
7.2
2.8
1.9
1.4
2.1
2.3
a-including the hyperfine components
Methylindoles – Microwave Spectroscopic Study

7. Theoretical Calculations
Electrostatic potential mapped onto a 0.20 a.u. electron density isosurface for the optimized structures, calculated at MP2/ G(d,p) level
1-methylindole
2-methylindole
3-methylindole
2 measurable dipole moment
rotor length?
4-methylindole
5-methylindole
6-methylindole
7-methylindole
Methylindoles – Microwave Spectroscopic Study

8. Structure from Rotational Constants
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHz
(15)
(13)
(4)
(6)
(5)
(9)
(13)
B/MHz
(7)
(3)
(2)
(4)
(12)
(2)
(11)
C/MHz
(4)
(2)
(2)
(9)
(5)
(9)
(7)
Pcc/u Å2
1.690
1.666
1.633
1.629
1.656
1.638
ΔJ/kHz
0.041(4)
0.0105(15)
0.048(2)
0.0409(11)
0.0145(5)
0.0035(7)
0.043(3)
ΔJK/kHz
-0.06(2)
0.02(2)
-0.072(7)
fixed 0.00
0.030(6)
-0.020(8)
δJ/kHz
0.013(2)
0.0012(11)
0.0156(7)
0.0158(7)
0.0037(2)
0.0172(9)
δK/kHz
0.11(3)
0.117(11)
0.053(6)
0.044(13)
-0.34(3)
0.042(9)
Dpi2J/MHz
0.048(4)
0.17(6)
0.056(5)
(2)
(4)
0.17(4)
Dpi2K/MHz
-0.15(7)
-0.5(2)
-0.097(12)
0.399(4)
0.397(6)
-0.29(6)
Dpi2-/MHz
0.021(5)
0.040(5)
(2)
(5)
0.16(4)
χaa/MHz
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
V3/cm-1
277.1(2)
374.32(4)
414.(5)
331.6(2)
(15)
(4)
426(3)
∠(i,a)/º
50.39(13)
54.(2)
65.29(7)
16.671(6)
18.118(9)
71.3(6)
∠(i,b)/º
39.61(13)
fixed 90.00
35.(2)
24.71(7)
73.328(6)
71.882(9)
18.7(6)
No. A linesa 80 79 86 95 63 83
No. E linesa 72 74 75 88 57 87
Δνrms/kHz
7.2
2.8
1.9
1.4
2.1
2.3
ΔIrms/u Å2
1.628
2.269
1.947
1.677
2.029
2.159
1.719
Methylindoles – Microwave Spectroscopic Study

9. Methylindoles – Microwave Spectroscopic Study
Quadrupole Analysis
parameter
Indolea
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
χaa/MHz
1.726
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.031
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
θ/ degrees
18.1
38.0
19.2
4.1
41.6
29.6
5.9
3.5
χcc / MHz
-3.379
(13)
-3.50 (6)
(7)
(9)
(5)
-3.39 (2)
(7)
χparallel / MHz
1.644
2.1 (2)
1.757 (1)
1.713 (1) -
1.634 (1)
1.679 (1)
1.618 (2)
χperpen / MHz
1.735
1.5 (2)
1.738 (1)
1.783 (1)
1.758 (1)
1.707 (1)
1.698 (1)
iσ (N-H)
0.24
0.32 (4)
0.25 (1)
0.24 (1)
0.23 (1)
πc (Pz)
0.38
0.32 (2)
0.36 (1)
0.38 (1)
0.37 (1)
0.39 (1)
c- (N)
0.36
0.50 (6)
0.35 (1)
a Suenram, R. D.; Lovas, F. J.; Fraser, G. T. Microwave Spectrum and 14N Quadrupole Coupling Constants of Indole. J. Mol. Spectrosc. 1998, 127,
Methylindoles – Microwave Spectroscopic Study

10. Methylindoles – Microwave Spectroscopic Study
Quadrupole Analysis
parameter
Indolea
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
χaa/MHz
1.726
1.730(9)
1.74(5)
1.783(5)
1.699(5)
1.728(4)
1.707(15)
1.698(5)
χbb-χcc/MHz
5.031
5.455(10)
5.25(3)
5.209(5)
4.993(8)
5.056(3)
5.066(9)
4.935(5)
θ/ degrees
18.1
38.0
19.2
4.1
41.6
29.6
5.9
3.5
χcc / MHz
-3.379
(13)
-3.50 (6)
(7)
(9)
(5)
-3.39 (2)
(7)
χparallel / MHz
1.644
2.1 (2)
1.757 (1)
1.713 (1) -
1.634 (1)
1.679 (1)
1.618 (2)
χperpen / MHz
1.735
1.5 (2)
1.738 (1)
1.783 (1)
1.758 (1)
1.707 (1)
1.698 (1)
iσ (N-H)
0.24
0.32 (4)
0.25 (1)
0.24 (1)
0.23 (1)
πc (Pz)
0.38
0.32 (2)
0.36 (1)
0.38 (1)
0.37 (1)
0.39 (1)
c- (N)
0.36
0.50 (6)
0.35 (1)
a Suenram, R. D.; Lovas, F. J.; Fraser, G. T. Microwave Spectrum and 14N Quadrupole Coupling Constants of Indole. J. Mol. Spectrosc. 1998, 127,
Methylindoles – Microwave Spectroscopic Study

11. Barrier to Methyl Internal Rotation
molecule
this work
fitted barrier
this work theoretical barrier
ωB97XD/ G(d,p)
Sinha et al.5
B3LYP/6-31G(d,p)
other experimental works
V3 (cm-1)
V6 (cm-1)
1-MI
277.1(2)
281.5
-28.6
274
282.8a
2-MI
374.32(4)
359.2
-26.5
348 -
3-MI
414.(5)
409.2
-25.8
419
443.2b, 500(40)c
4-MI
331.6(2)
338.0
-22.4
335
5-MI
(15)
125.1
-10.4
109
132.7a, 135(6)c
6-MI
(4)
127.1
-12.9
104
123.1a
7-MI
426(3)
436.7
-32.3
418
a Bickel, G. A.; Leach, G. W.; Demmer, D. R.; Hager, J. W.; Wallace, S. C. The Torsional Spectra of Jet‐Cooled Methyl Substituted Indoles in the Ground and First Excited States. J. Chem. Phys. 1987, 88, 1-8.
b Sammeth, D. M.; Siewert, S. S.; Callis, P. R.; Spangler, L. H. Methyl Rotor Effects in 3- and 5-Methylindole. J. Phys. Chem , 96,
c Remmers, K.; Jalviste, E.; Mistrik, I.; Berden, G.; Meerts, W. L. Internal Rotation Effects in the Rotationally Resolved S1(1Lb) ← S0 Origin Bands of 3-Methylindole and 5-Methylindole. J. Chem. Phys , 108,
Methylindole Electronic Environment – Microwave Spectroscopic Study

12. Barrier Forming Factors
NBO analysis – Sinha, R. K.; Singh, B. P.; Kundu, T. Origin of Threefold Methyl Torsional Potential in Methylindoles. Theor. Chem. Acc , 121,
1-MI
non-local hyperconjugative interactions
2-MI, 3-MI
local hyperconjugative interactions
4-MI, 5-MI, 6-MI, 7-MI
local structural distortions
Methylindoles – Microwave Spectroscopic Study

13. Methylindoles – Microwave Spectroscopic Study
Acknowledgement
Prof. Heinrich Maeder
Prof. Sean Peebles
Prof. Rebecca Peebles
Ohio Supercomputer Center
Kent State University
Methylindoles – Microwave Spectroscopic Study

15. Theoretical Calculations
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHza
2635.3
3722.2
2592.3
2154.4
3444.3
3400.9
2130.0
B/MHza
1301.8
985.8
1263.1
1479.6
1029.6
1037.5
1510.0
C/MHza
876.2
785.4
853.9
882.1
796.6
799.0
888.6
μa/Da
2.23
2.20
-0.93
1.82
1.41
-0.65
0.51
μb/Da
-1.04
-1.45
1.72
0.39
1.17
-1.81
-2.27
μc/Da
0.00
0.09
0.12
0.04
0.10
V3/cm-1 b
281.5
359.2
409.6
338.0
125.1
127.1
436.7
V6/cm-1 b
-28.6
-26.5
-25.8
-22.4
-10.4
-12.9
-32.3
rotor length/Åa
1.449
1.494
1.498
1.506
1.511
1.510
1.505
Methylindoles – Microwave Spectroscopic Study