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Physical Methods 8.3-8.5 Ultraviolet-visible spectroscopy, Infrared and Raman Spectroscopy, and Nuclear Magnetic Resonance

Jason Kuang

Absorption Spectroscopy

Physical technique to investigate inorganic compounds

Ultraviolet-visible spectroscopy

Infrared and Raman spectroscopy

Frequency can provide information on energy levels

Intensity can provide quantitative analytical information

Nondestructive and recoverable

Absorption Spectroscopy

Covers a full range of atomic and molecular energies – Ionization, Vibration, Rotation, Nuclear Reorientation

X and Ultraviolet – Electronic structures of atoms and molecules

Infrared – Vibrational behavior

Radiofrequency – Nuclear Magnetic Resonance, energies of reorientation of nucleus in magnetic field

Absorption Spectroscopy

Can affect the structural information that is collected

Consider lifetime and how a molecule may change during that interval

For example: IR takes a quicker snapshot of molecular structure than NMR

Ultraviolet-visible Spectroscopy

Absorption of electromagnet radiation in the UV and visible region

Electronic spectroscopy

Solution, gas, or solid – Usually a solution

Measures the intensity of light

Absorbance is calculated using the intensity of absorption

Ultraviolet-visible Spectroscopy

Ultraviolet-visible Spectroscopy – Beer-Lambert Law

Type of transition can be inferred from the molar absorption coefficient

Absorbance and concentration provides a way to measure properties that depend on concentration – equilibrium compositions and rates of reactions

Ultraviolet-visible Spectroscopy – Beer-Lambert Law

Source: http://public.iorodeo.com/docs/colorimeter/lab_2.html

Ultraviolet-visible Spectroscopy - Spectrophotometry

Measuring intensities rather than the energies of transitions

Spectrophotometric titration – reaction monitored by measuring the concentration

Monitor progress of reactions and determining rate constants

Can be applied to reactions with half-lives of picoseconds to hours and days

Can give information on the # of species that form in the reaction

Infrared and Raman Spectroscopy (Vibrational)

Complementary – vibration from one type may be observed in one but not the other

Characterize compounds – Strength, Stiffness, # of bonds

Fingerprinting – detect the presence of known compounds

Monitor changes in concentration of a species during a reaction

Determine components of an unknown compound (presence of a ligand)

Determine a likely structure

Measure properties of bond (force constants)

Infrared and Raman Spectroscopy (Vibrational) – Energies of Molecular Vibrations

Bonds stretch like strings , is the force constant

Stiffer the bond=↑force constant

Harmonic Oscillator, Solution of Schrödinger equation,

, μ=effective mass, , if then vibrational energy levels depend on

So, frequency is high when the effective mass of the oscillator is low and when the force constant is large

Vibrational energies expressed in terms of wave numbers, 300-3800 cm-1

Infrared and Raman Spectroscopy (Vibrational) – Energies of Molecular Vibrations

Molecules with N atoms vibrate in 3N-6 different ways (nonlinear) or 3N-5 different ways (liner)

Bending lower frequencies than stretching

IR active corresponds to a changing electric dipole moment

Raman active corresponds to a change in polarizability

Infrared and Raman Spectroscopy (Vibrational) – Techniques

Expose sample to infrared radiation  Record the variation of absorbance with frequency, wavenumber, or wavelength Spectrum

Glass cannot be used and aqueous solutions are not suitable

Optical windows are constructed with CsI or KBr disc are used

Raman – sample is exposed to intense laser radiation in the visible region

Photons are scattered and some are scattered inelastically due to losing energy to excite vibrations, which differ form the incident radiation by amounts equal to the vibrational frequencies

Aqueous solutions can be used, but linewidths are much greater

Resonance Raman Spectroscopy – species is colored, excitation laser is tuned to a real electronic transition

Infrared and Raman Spectroscopy (Vibrational) – Applications

Determination of the shape of an inorganic molecule

By the # of IR active mode and # of Raman active modes

Study of d block compounds with carbonyl ligands

Free CO abrosb at 2143 cm-1 but lowers when attached to a metal atom

Lowered by an amount depending on the extent which electron density is transferred

Distinguish terminal and bridging ligands (occurs at lower frequencies)

Matrix Isolation – Study molecules that are formed and trapped in inert matrices

Highly unstable species

Infrared and Raman Spectroscopy (Vibrational)

Nuclear Magnetic Resonance

Determine molecular structures in solution and pure liquids

Provide information on

Shape and symmetry

Rate and nature of interchange of ligands in fluxional molecules

Nondestructive

Sensitivity depends on the abundance of isotopes and large magnetic moment

1H larger abundance than 13C

Nuclei with even atomic # and even mass # have zero spin and invisible in NMR

12C and 16O vs. 1H, 13C, 19F, 31P

Nuclear Magnetic Resonance

Energy Separation  , =magnitude of the applied magnetic field, =magnetogyric ratio/ratio of its magnetic moment to its spin angular momentum

The low sensitivity of NMR can be increased using a stronger magnetic field

Nuclear Magnetic Resonance

Nuclear Magnetic Resonance – Chemical Shifts

Expressed in terms of chemical shift

δ=chemical shift

v=resonance frequency of nuclei in the sample

v0=resonance frequency of reference compound

Common standard is Tetramethylsilane Si(CH3)4

Nuclear Magnetic Resonance – Spin-spin Coupling

Structural assignment is helped by spin-spin coupling

Multiplets

Interactions of nuclear spins

Orientation of spin of a nearby nuclear affects the energy of another nucleus

Causes small change in location

Heteronuclear spins – coupling of nuclear spins of different elements

Homonuclear spins – coupling of nuclear spins of same elements

Size of coupling constants are related to the geometry of a molecule

Nuclear Magnetic Resonance

The intensity of a signal from a group of chemically equivalent nuclei is proportional to the number of nuclei in the group

NMR is slow in comparison to others

Temperature can be changed

NMR spectra of solid rarely show the same high resolution

Used for determining environments of atoms in a molecule (natural vs synthetic)

Enhance resolution by decoupling hetero and homonuclear, and use multiple pulse sequences