IR and NMR Spectroscopy

1
IR and NMR Spectroscopy

• Gaussian Workshop Day 4

2
Restricted vs. Unrestricted Redux

• Gaussian Defaults Split.
• RHF and RDFT for Closed Shell
• UHF and UDFT for Open Shell and Excited States
• Pg 10 in Foresman and Frisch
• Reason has to do with differences in magnetic
  environment
• Consider Be 1s22s2 vs. 1s22s12p1

3
Basics of Spectra

• Optimize First!
• Require only one word command NMR or Freq.
• Extraction of Spectra for Publication requires
  program like SWizard.
• UV/Vis spectra not so simple.

4
NMR Basics

• Do not need to optimize with same basis as you
  run NMR
• Must compare with TMS Si(CH3)4 for relative
  shifts in same Basis/Method

5
IR Spectra

• Must Optimize with Same Basis Set/Method
• Calculation must be fully optimized or
  imaginary/negative frequencies will result.
• Also gives thermochemical information for 1Atm
  and 298K.

6
IR Spectra Accuracy

• Calculations rely on Harmonic approximation
• Often overestimate frequencies as a result
• Results usually scaled
• Scale factors dependent on both method and basis

7
Isotopes (From Help File)

• C(Iso13,Spin3) 0.0 0.0 0.0
• The line specifies a 13C atom with a nuclear spin
  of 3/2 (3 1/2), located at the origin. The
  following items may be included in the list of
  parameters
• Ison Isotope selection. If integers are used to
  specify the atomic masses, the program will
  automatically use the corresponding actual exact
  isotopic mass (e.g., 18 specifies 18O, and
  Gaussian uses the value 17.99916).
• Spinn Nuclear spin, in units of 1/2.
• ZEffn Effective charge. This parameter is used
  in spin orbit coupling (see CASSCFSpinOrbit),
  and the ESR g tensor and the electronic
  spin-molecular rotation hyperfine tensor (NMR
  OutputPickett).
• QMomn Nuclear quadrupole moment.
• GFacn Nuclear g-factor.

8
Homework

• Read Introduction to Part 2, Chapters 5/6.
• Get comfortable with the basics
• Solvation next? Or move onto Part 2?