generalchem_ls_18.pdf

8/10/2019 GeneralChem_LS_18.pdf

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BITSPilani, Pilani Campus

CHEM F111 General Chemistry

BITSPilaniPilani Campus

Dr. Ajay K. Sah


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Topic Discussed


The nuclei that can be used for NMR1H NMR

Number of possible signals

Chemical shift

Anisotropic effect

Spin-spin coupling; multiplicity of signal

Integration of signal


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Spin-spin coupling: Examples


A doublet (1:1) for methyl hydrogens and a quartet

(1:3:3:1) for the aldehydic hydrogen

The central H atom has six equivalent neighbours

and one neighbour for these six equivalent hydrogens

A heptate of ratio 1:6:15:20:15:6:1 for the centralhydrogen and a doublet for methyl hydrogens.

Ratio of the total areas under the peaks would be 1:6

CH3CHO

CH3CH(Cl)CH3


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Spin-spin coupling constant


NJXY

X is the nucleus of interest

N = position of coupled nucleus Y

Example1

JCH means the coupling13

C-1

H.2JCH means the coupling

13C-C-1H.

1J - 102to 103Hz

2J - 10 to 102Hz

3

J and4

J are low but detectable.


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Spin-spin coupling


Karplus equation: 3JHH

= A+ B cos+ C cos2

( is the dihedral angle HCCH, typical values ofA,

B and C are, +7, -1 and +5 Hz respectively)

1JCH depends on the hybridization of C atom.

1JCH/ Hz 250(sp) 160 (sp2) 125 (sp3)

Spin-spin coupling is through bonds via

Polarization mechanism.


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Spin-spin coupling


Polypeptide conformation:


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Spin-spin coupling constant


Since, Espin-spin

= hJAX

mAm

X;1JCH

+ ve (anti

parallel arrangements are low energy)

1) Dipolar interaction

2) Fermi contact interaction ( requires s-electrondensity)

Low energy : XeeY

High energy : Xe

eYOR

Low energy : X e. eY

High energy : Xe.

eY


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9/15BITSPilani, Pilani Campus

1H NMR of Vinyl acetate

HD HB HC

3JBD

3JCD

3

JCD3JBD

2JBC

2JBC

trans trans

cis

cis

gem gem

CH3 C

O

O

C

HD

C

HC

HB

3J-trans > 3J-cis > 2J-gem

A



1H NMR of Vinyl acetate


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1H-NMR spectroscopy


Problem 1Molecular formula: C5H10O

IR (cm-1): 2900 (s), 1710 (s)

1H NMR (): 2.7 (1H, Septet), 2.2 (3H, S), 1.1 (6H, d)

Problem 2

Molecular formula: C6H10O3

IR (cm-1): 2900 (s), 1710 (s)

1H NMR (): 4.1 (2H, q), 3.3 (2H, S), 2.2 (3H, s),

1.2 (3H, t)



Thermodynamics

From the Greek thermos meaning heat and

dynamics meaning power

Study of energy and its transformations, in

particular, the inter conversion of heat and work.



Study of the effects of changes in temperature,pressure, and volume on physical systems at the

macroscopic scale by analyzing the collective

motion of their particles using statistics.

It tells us about

Heat output of a chemical reaction.

Work output from a chemical reaction. Why a particular reaction reaches equilibrium?

What is the equilibrium composition of a reaction?

Thermodynamics



Thermodynamics

Classical thermodynamics: independent of anymodel of the microscopic structure of matter.

Statistical Thermodynamics: provides link betweenmicroscopic structure and macroscopic

thermodynamic properties.


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Thermodynamics

The First Law

Thermochemistry

The Second LawThe Third Law

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