Presented by Santosh Kumar Singh Integrated PhD student Supervisor Dr Aloke Das 1 Lone pair of Oxygen of carbonyl group or any lone pair containing atom interacts with the aromatic ring ID: 630684
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Slide1
n→𝜋* non-covalent interaction is weak but strong in action
Presented by- Santosh Kumar Singh
Integrated Ph.D studentSupervisor – Dr. Aloke Das
1Slide2
Lone pair of Oxygen of carbonyl group or any lone
pair containing atom interacts with the aromatic ring.
Lone pair of oxygen of carbonyl group
interacts with the near by carbonyl group.
Definition of n
→𝜋*
interaction
1. n
→𝜋*
am
Burgi-Dunitz
trajectory
d
: O...C=O distance 3.2 Å : O...C=O = 109 ± 100
(D): 2.8-3.8 Å
() : 900
Blackwell and coworkers, J. Am. Chem. Soc. 2007, 129, 8928-8929.
M.
Egli
and S.
Sarkhel
, Acc. Chem. Res., 2007, 40, 197–205.
2. n
→𝜋*ar
cis-amide
trans-amide
2
It seems to be
counterintuitiveSlide3
Hydrogen bonding vs.
n
* interaction
Hydrogen bonding (N-H…N)
(n
→𝜎*)
n
𝜋*
n
*
Das and co-workers, J. Phys. Chem. A, 2012, 116, 11573.
N-
trimethylacetyl
homoserine lactoneRaines and co-workers, ACS Chem. Biol., 2014, 9,880–883.𝜎*n
3
Hydrogen bondingn
* Slide4
Structure of Z-DNA
α
- helix protein
Nat. Chem. Biol.,
2010
, 6, 615-620
Collagen triple helix
Cytosine
Guanine
J. Am. Chem. Soc. 2002, 124, 2497
4
n
* interaction in biologySlide5
n
* interaction in Material science
CrystEngComm
, 2008,
10, 1501-1515Slide6
Study of n
→𝜋* interaction from NMR spectroscopy
CIS
Trans
J. Am. Chem. Soc. 2009, 131, 7244.
J. Am. Chem. Soc. 2013, 135, 7843.
6Slide7
IR
IR
ν
cm
-1
3521
ν
cm
-1
3450
3521
7-azaindole
Probe N-H stretching frequency for N-H..N hydrogen bonding
Probe carbonyl stretching frequency for
n
* interaction
IR
7
Signature of
n
→𝜋* interaction from IR spectroscopy? Slide8
Electronic spectrum of phenyl formate in solution of
acetonitrile
Optimized geometries of phenyl formate calculated at the M05-2X/
aug
-cc-
pVDZ
level of theory
Two conformers of phenyl formate cannot be identified from solution phase electronic spectrum
8
Phenyl formateSlide9
Experimental setup and computational methods
Home-built Jet-cooled
REMPI
(Resonantly
Enhanced
Multiphoton
Ionization
) Time Of
Flight Mass
Spectrometer
Mass selected electronic and
vibrational
spectroscopy of molecules and complexes
Computational methods: MP2, M05-2X, wB97-D, M06-2X, CIS
DFT-D level of calculation using Gaussian G099Slide10
100 ns
Pump UV
Probe UV
S
0
S
1
D
0
UV
10
Resonant two photon ionization
(1C-R2PI)
UV-UV
holeburn
spectroscopy
(37673 cm
-1
)
(37613 cm
-1
)
43
86
S. K. Singh, K. K.
Mishra
, N. Sharma, and A. Das,
Angew
. Chem. Int. Ed.
2016, 55, 7801
Conformation-specific electronic spectroscopy of phenylformateSlide11
IR-UV double resonance spectroscopy
11
Thus direct spectroscopic evidence of
n
* interaction is obtained for the first time through IR-UV double resonance spectroscopy.
S. K. Singh, K. K.
Mishra
, N. Sharma, and A. Das,
Angew
. Chem. Int. Ed.
2016, 55, 7801
Conformation-specific IR spectroscopy of phenyl formateSlide12
Natural Bond Orbital Analysis
NBO view of
n
* interaction :
Overlap of oxygen lone pair (n) and
* orbital of ring.
Cis conformer
Trans conformer
12
S. K. Singh, K. K.
Mishra
, N. Sharma, and A. Das,
Angew. Chem. Int. Ed. 2016, 55, 7801 Slide13
Conclusions
Direct spectroscopic evidence for an
n
* interaction has been obtained for the first time by probing C=O stretching frequency in the gas phase.
Although n* interaction is very weak in nature, it can provide conformational
preference i.e. the cis conformer of phenyl formate is more stable than the
trans conformer due to the presence of the n
* interaction in the former one.
13
n
* interaction govern the conformational preferences of salicin even when strong hydrogen bonding interaction is present nearby. Slide14
ACKNOWLEDGEMENT
Dr. Aloke Das for his suggestions and help to carry out the project.
My lab members for supporting me during the project.
DST-SERB and Infosys foundation for International travel support.
IISER Pune for my fellowship and lab facilities
.
Organizers of ISMS-2017 for providing me the platform to present my work.
14Slide15
15Slide16
C=O lone pair-
* orbital (phenyl ring) interaction and red-shift in C=O freq
Resonance effect reduces
C=O stretch frequency
Inductive effect increases
C=O stretch frequency
Resonance
effect
Inductive
effect
n
*
Presence of the n
* interaction in the cis
conformer favors the neighboring orbital interactions which reduce the C=O stretching frequency
12
13
14
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6
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5
4
9
3
2
8
7
1
12
13
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15
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6
10
5
4
9
3
2
8
7
1
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6
10
5
4
9
3
2
8
7
1
R
1
R
2
R
3
NBO analysis