Effect of ring size effect on ir spectra of carbonyl carbon
Answers
The Nature of Vibrational Spectroscopy
We have noted that the covalent bonds of molecules are not rigid , but are more like stiff springs that can be stretched and bent. At ordinary temperatures these bonds vibrate in a variety of ways, and the vibrational energies of molecules may be assigned to quantum levels in the same manner as are their electronic states. Transitions between vibrational energy states may be induced by absorption of infrared radiation, having photons of the appropriate energy. It requires more energy to stretch (or compress) a bond than to bend it, and as might be expected, the energy or frequency that characterizes the stretching vibration of a given bond is proportional to the bond dissociation energy.
The equation on the right describes the major factors that influence the stretching frequency of a covalent bond between two atoms of mass m1 and m2 respectively. The force constant (f) is proportional to the strength of the covalent bond linking m1 and m2. In the analogy of a spring, it corresponds to the spring's stiffness. For example, a C=N double bond is about twice as strong as a C-N single bond, and the C≡N triple bond is similarly stronger than the double bond. The infrared stretching frequencies of these groups vary in the same order, ranging from 1100 cm-1 for C-N, to 1660 cm-1 for C=N, to 2220 cm-1 for C≡N.
Approximate Infrared Stretching Frequencies
B-H
2400 cm-1
C-H
3000 cm-1
N-H
3400 cm-1
O-H
3600 cm-1
F-H
4000 cm-1
Al-H
1750
Si-H
2150
P-H
2350
S-H
2570
Cl-H
2890
Ge-H
2070
As-H
2150
Se-H
2300
Br-H
2650
If one of the bonded atoms (m1 or m2) is a hydrogen (atomic mass =1), the mass ratio in the equation is roughly unity, but for two heavier atoms it is much smaller. Consequently, C-H, N-H and O-H bonds have much higher stretching frequencies than do corresponding bonds to heavier atoms. Other X-H stretching frequencies are shown in the table to the left, the trends observed being due chiefly to differences in the force constants. The mass effect on stretching frequencies is particularly evident when deuterium isotope equivalents are compared with corresponding hydrogen functions. Thus, the stretching frequency of a free O-H bond is 3600 cm-1, but the O-D equivalent is lowered to 2600 cm-1. Since deuterium has a mass = 2, the mass term in the equation changes fron 1 to 1/2, and the frequency is reduced by the square root of 2. In this discussion we have focussed on stretching vibrations, and it should be noted that bending vibrations may be treated in a similar fashion.
Not all molecular vibrations lead to observable infrared absorptions. In general, a vibration must cause a change in the charge distribution within a molecule to absorb infrared light. The greater the change in charge distribution, the stronger the absorption.