CCL: Isotopic Vibrational Frequencies in g09

 Sent to CCL by: "Dom  Esan" [desan2]![]
 Good day all,
 I ran some vibrational frequency calculations using different atomic isotopes in
 g09 but I'm getting some "weird" results.
 Basically, I used the checkpoint file from a previous frequency calculation of a
 non-substituted molecule (CH2NH2, say) and then used "readfc and
 readisotopes" to calculate the frequencies of substituted isotopes of C or
 N (13C or 15N). The resultant frequencies from the isotopic calculations were
 *higher* than those of the non-substituted molecule(s). One would expect the
 frequencies of the heavier isotopes to be *lower*.
 Does anyone have any suggestions as to what might be wrong?
 All calculations were done on a metal cluster with the metal atoms frozen.
 Thank you for your time and anticipated assistance.