The infrared spectrum for tris(pyrazolyl)borate (TptBu,iPr)Cu(NO) is shown above
ID: 987270 • Letter: T
Question
The infrared spectrum for tris(pyrazolyl)borate (TptBu,iPr)Cu(NO) is shown above...
It is believed the band at 1698 cm-1 is due primarily to the NO stretch. Some accurate isotopic masses are: 14N 14.0031 amu 15N 15.0001 amu 1 amu = 1.66054 x 10-27 kg Added 16O 15.99491 amu 18O 17.99916 amu ...
a) Calculate the force constant (in N m-1) for this vibration, and comment on the nature of the NO bond in this complex ?
b) In order to test assignment of this peak at 1698 cm-1 in the IR spectrum, the authors prepared the doubly isotopically substituted 15N18O complex. In the IR spectrum for the 15N18O copper complex, they found that the 1698 cm-1 peak had disappeared and a new band was observed at 1625 cm-1. Is the authors’ assignment correct? Explain your decision with a calculation.
Explanation / Answer
a) reduced mass (u) for NO = m1.m2/m1+m2
= (14.0031 x 15.99491) amu/((14.0031 + 15.99491) amu
= 7.46644 amu = 7.46644/6.023 x 10^26
= 1.24 x 10^-26 kg
force constant (k) = (v.2.pi)^2.reduced mass
= (1698 cm-1 x 3 x 10^10 cm/s x 2 x 3.24)^2.1.24 x 10^-26 kg
= 1269.376 kg/s^2
b) reduced mass for isotopic NO = (15.0001 x 17.99916) amu/(15.0001 + 17.99916) amu
= 8.1817/6.023 x 10^26
= 1.36 x 10^-26 kg
frequency (v) = 1/2 x 3.14[sq.rt.(1269.376 kg/s^2/(1.36 x 10^-26))]
= 4.865 x 10^13 s-1/3 x 10^10 cm/s
= 1622 cm-1
So the assumption for NO peak shifting at 1625 cm-1 is approximately correct in this case. Actual value 1625 cm-1, calculated value comes arounf 1622 cm-1.
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