Physical Chemistry: Spectroscopy

Beer-Lambert

Use the ________ law to calculate concentration, molar extinction coefficient, path length, the intensity of incident light, the intensity of transmitted light or absorbance.

electromagnetic radiation

In spectroscopy, we analyze the ________ (light) absorbed, emitted, or scattered by matter consisting of molecules or atoms.

Convert energy

________ from Joules to wavelength (m, cm, nm or Å), to wavenumber (cm- 1) or to frequency (Hz or s- 1) using Plancks constant and /or the speed of light.

Spectroscopic investigations

________ give us information about the structure of the molecules of a sample or its chemical composition.

The relationship between frequency, wavelength and velocity

Quantum theory

________ states that the energy levels in atoms and \n molecules are quantized in specific allowed energy levels (i. e. discontinuous). The energy difference between two levels E1 and E2 can be measured by the absorption of emission of light using photons with the energy difference DE.

Beer-Lambert Law

Selection Rule

a statement about when a transition is allowed or forbidden, because not all transitions that fulfill the resonance condition can be observed. Some of them are not allowed for symmetry reasons

Spectrometer

detects the light absorbed, emitted, or scattered by a sample. It consists of a light source, a dispersing element, and a detector

Radiofrequency region

\
Electromagnetic radiation used for NMR spectroscopy typically lies in the _____________.

quantized spin angular \n momentum

Any atomic nucleus which possesses either odd mass number, odd atomic number or both, has a ____________, I, and since it is positively charged, a magnetic moment, μ, associated with it.

Common nuclei that possess spin are

1 H (I=1/2), 13 C (I=1/2), 19 F (I=1/2), 2 H(I=1), 31 P (I=1/2), 14 N (I=1)), and two common nuclei that are not magnetic are 12 C (I=0) and 16 O (I=0)

NMR active nuclei can be in

2I + 1 possible quantized spin states, which only fit either the integer of half-integer sequence

1H and 13C nuclei only have 2 active NMR

m= +1/2 and m= -1/2

nuclear magnetic resonance (NMR)

is the observation of the frequency at which magnetic nuclei in \n molecules come into resonance with an electromagnetic field \n when the molecule is exposed to a strong magnetic field.

The great utility of NMR as a structure determination tool is due \n to two major effects felt by the magnetic nuclei:

1) the resonant frequency n varies due to local magnetic fields \n felt at the nucleus (called chemical shift, d) and \n 2) splitting of resonance into fine structure due to neighboring \n magnetic nuclei (called J-coupling or spin-spin \n coupling).

Shielding and deshielding depend on

electronegativity of neighbouring atoms

The number of signals in the 1H NMR spectrum is dependent on

how many kinds of chemically nonequivalent protons the compound has.

chemically equivalent protons

two nuclei are equivalent if they are interchangeable by a symmetry operation

chemical shift

depends on the electron density around the nucleus, it is responsive to the effects of electronegativity in the bonds and therefor the amount of shielding

proportional to the number of protons

The area under each signal is _____________ that cause the absorption

relative number ; absolute number

The integration tells us the _________ of protons that give rise to each signal, not the __________

Libile H’s (e.g. -OH, -NH, etc)

\
can exchange with solvent, this leads to line broadening in the 1H NMR spectra. Splittings due to other nuclei are not observed.

The splitting of a signal in H-NMR

is described by the N+1 rule, where N is the number of equivalent protons bonded to an adjacent carbon

proton AX spin systems

if coupled always gives a pair of doublets. The two orientations of the magnetic field of the Hx proton causes the Ha resonance to split into a doublet of peaks. Likewise, the Hx resonance is also split into a doublet by Ha’s two spin states

spin coupling constant, J

The frequency distance in Hz between the peaks in each doublet, can take on a range of values from \~20Hz to

Larmor frequency

depends on the value of the magnetic field at the nucleus, induced by the applied field Bo due to deshielding efect depending on the orientation of the local induced field. Increases linearly with Bo.

AX2 spin system

an Ha proton is split by two equivalent Hx protons and the Hx resonance will split into a doublet just as it is jut above the AX system. The Ha resonance will however be split into a 1:2:1 triplet due to probabilities of the spin orientation of two Hx nuclei.

Pascal’s Triangle

The relative intensity of the lines in the multiplet in NMR is given by the “binomial coefficients of order n “ also known as ________

three σ

Protons normally do not split each other if they are separated by more than ________ bonds.

number of distinct 13C environments

The numbers of resonances visible in a 13C NMR spectrum \n immediately indicates the ______ in the molecule. If the number of 13 C resonances is less than the number in the molecule, then the molecule must have some symmetry that dictates this.

The areas under

_________ the 13C singlets in 13C NMR do not correspond to the number of carbons present in the molecule; consequently integration of decoupled carbon spectra is not done.

Broadband decoupling

Decoupling 1H from 13C resonance, so that each distinct 13C environment gives rise to a separate singlet peak. Dramatically improves the signal/noise of the 13C resonance peak.

Distortionless Enhancement by Polarization Transfer (DEPT)

is a double resonance pulse program that transfers polarization from an excited nucleus to another – most commonly 1H → 13C. This results in a sensitivity enhancement relative to the standard decoupled 1D carbon spectra

Ultraviolet (UV) / Visible (VIS) Spectroscopy

In UV/VIS spectroscopy transitions between different electronic states of molecules are considered. By absorption or emission of light electrons are transferred between energetically different energy molecules. UV/VIS spectra often give broad spectra because electronic transitions are accompanied by many vibrational and rotational transitions.

electronic absorption spectrum

Tell if an organic molecule has conjugated double bonds \n from an ______________

wavelength of maximum absorption

Estimate the length of a conjugated p system from the \n _____________ observed in the UV- \n Vis spectrum.

Molecular Orbital Models

give us a simplified picture of the types of electronic transitions that occur in UV/VIS spectroscopy.

Franck-Condon principle

Because nuclei are so much more massive than electrons, an \n electronic transitions takes place faster than the nuclei can respond. That means that the geometry of the nuclei does not change upon the transition. As outlined below it makes a statement about the intensity of electronic transitions.

Transition a

If one starts in the electronic ground state in the lowest vibrational state labeled v" the Franck-Condon principle says \n that the geometry does not change upon electronic \n excitation. This means one follows the arrow a (vertical transition) and ends up in the excited electronic state in the vibrational excited state labeled v'.

Chromophere

Use the term in UV/VIS spectroscopy to indicate a part of the molecule that gives rise to the UV/VIS electronic absorption

Examples of MOs that are chromopheres

carbon-carbon double bonds, extended conjugated \n series of double bonds, phenyl rings, isolated carbonyl groups or carbonyls conjugated with extended p systems

more conjugated double bonds are present

The ________, the lower is the energy of the transition and the longer the wavelength of the absorption

Types of radiative decay

\n - Fluorescence is an allowed transition from a singlet state to a singlet state. It is fast (ns). \n - Phosphorescence is a “forbidden” transition from a singlet state to a triplet state. It is slow (s).

Steps of Fluorescence

\
1\. Step: Excitation of a molecule from the singlet ground \n state an excited singlet state (Franck-Condon).

\n 2. Step: By collisions with neighbouring molecules \n part of the energy is discarded as heat to the \n surroundings (radiationless transition).

\n 3. Step: Radiative decay from the lowest \n vibrational state of the excited electronic state to \n an excited vibrational state of the electronic \n ground state (Franck-Condon).

\n 4. Step: Radiationless decay to the \n vibrational ground state.

Extended X-Ray Absorption Fine Structure (EXAFS)

Information about interatomic distances, coordination numbers, higher energy than UV

X-Ray Photoelectron Spectroscopy (XPS, ESCA)

chemical composition of the sample, higher energy than UV

stronger the absorption

The bigger the value E, the _______

The harmonic oscillator

The simplest model that describes molecular vibrations of \n diatomic molecules is the harmonic oscillator, in which a particle is restrained by a spring that obeys Hooke’s law

Hooke’s law

Restoring force = -kx (k is the force constant; \[k\] = \[N/m\])

gross selection rule

for vibrational spectra is that the electric dipole moment of the molecule must change during the vibration

Diple moment

the size of an electric dipole

Homonuclear diatomics

(e.g. O 2 or N2 ) are IR inactive since there can be no dipole moment change during their one and only stretch mode. So the vibrations neither absorb nor generate radiation.

Heteronuclear diatomics

will all be IR active since each will have a dipole that will change on stretching.

Common vibrational modes

* symmetrical stretching
* asymmetrical stretching
* scissoring (bending)
* rocking
* wagging
* twisting

Selection rules for infrared spectra of polyatomic molecules

depend on the symmetry of the molecules. These rules say which transitions are allowed (can be observed) and which transitions are forbidden (cannot be observed)

fingerprint region

contains all the possible bending vibrations of a molecule

Mass spectrometry

is used to determine the mass of atoms and molecules and for the structure determination of molecules through characterization of characteristic fragments

Ionization in Mass Spectroscopy

The molecules (M) can be ionized/fragmented by light (PI: \n photoionization) or by bombarding them with an electron beam (EI: electron impact ionization) having sufficient energy to ionize/fragment the molecules.

Mass selection in mass spectroscopy

can be done through magnetic field and by time of flight

labile bonds

Fragmentation of the most ________ leads to characteristic fragments. From these fragments structural information about the molecule can be obtained.