Electronic, Vibrational and Rotational Spectroscopy Flashcards

Flashcards for key vocabulary and concepts in Electronic, Vibrational, and Rotational Spectroscopy, based on lecture notes.

Spectroscopy

The study of how electromagnetic radiation interacts with matter through absorption, emission, or scattering.

Electronic Spectroscopy

How electrons move between orbitals, studied using spectroscopy.

Vibrational Spectroscopy

How molecules vibrate, studied using spectroscopy.

Rotational Spectroscopy

How molecules rotate, studied using spectroscopy.

Wavelength (λ)

Length of one wave.

Frequency (ν)

Number of waves per second; measured in Hertz (Hz).

Wavenumber (~ν)

Number of waves in 1 cm; measured in cm-1.

Photons

A stream of particles carrying a specific amount (a quantum) of energy.

Wave-Particle Duality

The principle that radiation or matter can appear to be waves or particles depending on how they are observed.

Quantization

A molecule is allowed to exist only in one of several discrete energy levels or states.

Transition

A molecule changes from one energy level/state to another, involving the intake or release of a discrete amount of energy.

Spectrum

A plot of absorption, emission, or scattering strength versus wavelength, wavenumber, or frequency.

Lines/Bands/Peaks

Regions where a molecule absorbs, emits, or scatters energy; can be narrow (lines) or broad (bands).

Resolution

Smallest peak separation that can be distinguished, determined by monochromator slit width.

Transmittance (T)

The fraction of incident light that is transmitted through a sample.

Absorbance (A)

The negative logarithm of the transmittance.

Selection Rules

Rules that state which transitions are allowed (give a strong band) and which are disallowed (forbidden).

Center of Gravity (C)

The balance point of a molecule, where ∑mi ri = 0.

Moment of Inertia (I)

Resistance to rotation; I = ∑mi ri^2.

Rotational Energy (E)

Kinetic energy due to rotational motion.

Angular Velocity (ω)

Measure of rotational speed.

Rotational Quantum Number (J)

Quantum number that quantizes rotational energy.

Rotational Constant (B)

Constant that depends on the molecule; B = h / (8π2Ic).

Centrifugal Distortion Constant

accounts for the effect of the changing bond length on the rotational energy levels

Gross Selection Rule (Rotational Spectroscopy)

For absorption or emission, there must be a change in dipole moment on rotation.

Specific Selection Rule (Rotational Spectroscopy)

ΔJ = ± 1; a transition is allowed only to the next rotational level.

Degrees of Freedom

The number of independent ways a molecule can move in space.

Hooke's Law

Restoring force exerted by a bond when stretched or compressed; f = -k(r - re).

Potential Energy (P.E.)

Energy due to the position of atoms in a molecule; P.E. = (1/2)k(r - re)^2.

Force Constant (k)

Stiffness of a spring or strength of a bond.

Vibrational Frequency (νvib)

Number of oscillations per second, independent of energy or amplitude.

Vibrational Wavenumber (~νvib)

Frequency in wavenumber units.

Vibrational Quantum Number (v)

Quantum number that quantizes vibrational energy.

Zero Point Energy

The lowest vibrational energy level, where v = 0; the molecule is not permitted to have no vibrational energy.

Gross Selection Rule (Vibrational Spectroscopy)

For absorption or emission, there must be a change in dipole moment on vibration.

Specific Selection Rule (Vibrational Spectroscopy)

Δv = ± 1; a transition is allowed only to the next vibrational level.

Born-Oppenheimer Approximation

The total energy is the sum of the individual electronic, vibrational, and rotational energies.

Skeletal Vibrations

Involve several or all atoms within a molecular skeleton; bands may occur in the fingerprint region (700-1500 cm-1).

Group Vibrations

Localized on a specific bond or group; give bands at similar wavenumbers regardless of the molecule.

Chromophore

A molecule or group that absorbs in the UV-visible region.

Color Wheel

If one color is absorbed, the complementary color is observed.

Beer-Lambert Law

A = εcl, relates absorbance to concentration and pathlength.

Isosbestic Point

Constant absorbance at a specific wavelength as two species interconvert; occurs when two interconverting species have the same ε at a specific λ.

Raman Spectroscopy

The study of how EM radiation interacts with a molecule to change its rotational or vibrational energy by inelastic light scattering.

Inelastic Light Scattering

Collision between a photon and a molecule where energy is exchanged.

Elastic Scattering (Rayleigh Scattering)

No change in energy of the photon or the molecule.

Inelastic Scattering (Raman Scattering)

Energy is exchanged between the photon and the molecule.

Gross Selection Rule (Raman Spectroscopy)

For Raman scattering, there must be a change in polarizability on rotation (rotational Raman) or vibration (vibrational Raman).

Polarizability

The ease with which the electrons within a molecule can be moved by an applied electric field.

Mutual Exclusion Rule

If a molecule has a center of inversion, its Raman active modes are IR inactive, and vice versa.