Organic Chemistry Spectroscopy Flashcards
___ is the process of measuring the frequencies of electromagnetic radiation (light) absorbed and emitted by a molecule
spectroscopy
___ measures molecular vibrations
infrared spectroscopy
The ___ of a sample is determined by passing infrared light through the sample and recording the frequencies at which the light is absorbed. The useful absorptions of infrared light occur in the frequency range 4000 - 400 cm-1 (called wavenumbers), which can be split into a functional group region and a fingerprint region. In the functional group region, the wavenumber of the absorption is typically related to the stretching vibrations of specific functional groups in a molecule and can be used to identify these functional groups. In the fingerprint region, the spectra, which usually consists of bending vibrations within a molecule, is complex and unique (like a fingerprint) for each compound.
infrared spectrum
___ is observed in the region of 4000-1500 cm-1, and bending vibrations are observed in the region of 1500-400 cm-1
bond stretching
In order for IR light to be ___, the vibration of the molecule must result in a change in a bond dipole moment. Symmetrical molecules do not have a dipole moment and therefore do not absorb IR light. The more complicated a molecule, the more peaks observed in its IR spectrum
absorbed
Alkanes 2800-3000 C-H 1200 C-C Alkenes 3080-3140 ||C-H 1645 C||C Alkynes 3300 |||C-H 2200 C|||C Aromatic 2900-3100 C-H 1475-1625 C-C Alcohols 3100-3500 O-H (broad) Ethers 1050-1150 C-O Aldehydes 2700-2900 (O)C-H 1725-1750 C||O Ketones 1700-1750 C||O Acids 2900-3300 O-H (broad) 1700-1750 C||O Amines 3100-3500 N-H (sharp)
Some of the most useful information in an ___ can often be quickly and easily determined by reviewing two key areas of absorption - from 1700-1750 and 3100-3500 cm-1 - to determine the presence of oxygen and nitrogen functional groups
Functional Group 1700-1750 cm-1 3100-3500 cm-1 Alcohol No Broad peak Aldehyde or Ketone Yes No Carboxylic Acid Yes Broad Peak Amine No Sharp Peak
IR spectrum
A great deal of information can be obtained from an IR spectrum. ___, the inverse of absorbance, is plotted versus frequency; this gives areas of absorption the appearance of valleys on the spectrum
percent transmission
___ is measured in the frequency range of 4000-400 cm-1, and is plotted as transmittance vs. wave number, with absorption appearing as “valleys” in the spectrum.
Different functional groups show IR absorption at different wave numbers. Some of the most useful absorption ranges to note are at 1700 cm-1 (C||O bond) and at 3100-3500 cm-1 (O-H if broad, N-H if narrow). However, other absorption frequencies an also be important
IR absorption
Certain ___ have magnetic moments that are normally oriented at random. When such nuclei are placed in a magnetic field., their magnetic moments tend to align either with or against the direction of the field. Nuclei whose magnetic moments are aligned with the field are said to be in the alpha-state (lower energy, while those whose moments are aligned against the field are said to be in the beta-state (higher energy)
nuclei
If the nuclei are then exposed to ___, some in the lower energy alpha-state will be excited into the beta-state; this transition is known as resonating. The absorption corresponding to this excitation occurs at different frequencies depending on the atom’s environment. Other nearby atoms in the same molecule (or sometimes in the solvent) can shift the frequency at which an atom transitions between the alpha and beta states. A compound may contain many nuclei that resonate at different frequencies, producing a very complex spectrum
electromagnetic radiation
Since different NMR ___ operate at different magnetic field strengths, a standardized method of plotting the NMR spectrum has ben adopted. A variable called chemical shift, represented by the symbol lowercase delta and with units of parts per million (ppm), is plotted on the x-axis. Chemical shift is compared to the standard organic molecule tetramethylsilane (TMS). TMS is always shown by a peak at 0 ppm, and the peaks of interest always appear to the left of TMS. A peak further to the left is said to be shifted downfield
spectrometers
Most H,1 nuclei come into ___ between 0 and 10 delta downfield from TMS. each distinct set of nuclei gives rise to a separate peak. The compound dichloromethyl methyl ether(Cl2HC-O-CH3) has two distinct sets of H,1 nuclei. The single proton attached to the dichloromethyl group is in a different magnetic environment compared to the three protons on the methyl group, and the two types of protons resonate at different frequencies. The three protons on the methyl group are magnetically equivalent, due to rotation about the oxygen-carbon single bond, and resonate at the same frequency.
resonance
If two magnetically different ___ are within three bonds of each other a phenomenon known as coupling or splitting occurs. Consider two protons, H sub a and H sub b, on the molecule 1,1,-dibtomo-2,2-dichloroethane.
At any given time, H sub a can experience two different magnetic environments, since H sub b can be in either the alpha or the beta state. These different states of H sub b influence nucleus H sub a, causing slight upfield and downfield shifts. Since there is a 50 percent chance that H sub b will be in either state, this results in a doublet, two peaks of equal intensity equally spaced around the true chemical shift of H sub a. H sub b experiences the two different states of H sub a an dis likewise split into a doublet. In 1,1-dibromo-2-chloroethane, the H sub a nucleus is affected by two nearby H sub b nuclei, which, taken together, can be in four different states: alpha-alpha, alpha-beta, beta-alpha, or beta-beta.
The alpha-beta and beta-alpha states have the same net effect on the H sub a nucleus, and the resonances occur at the same frequency. The alpha-alpha and beta-beta states resonate at frequencies different from each other and from the alpha-beta/beta-alpha frequency. The result is three peaks centered around the true chemical shift, with an area ratio of 1:2:1. The number of peaks that will appear when splitting occurs can be determined as follows: n hydrogen atoms couple to give n+1 peaks, whose area ratios are given by Pascal’s triangle.
protons
C, 13 NMR is very similar to H, 1 NMR. Most C, 13 NMR signals, however, occur 0-200 delta downfield from the carbon peak of TMS, so a spectrum for C, 13 NMR is easily recognized by the larger scale of the x-axis. Another significant difference is that only 1.1 percent of carbon atoms are C, 13 atoms. This has two effects: First, a much larger sample is needed to run a C, 13 spectrum (about 50 mg compared with 1 mg for H, 1 NMR), and second, coupling between carbon atoms is generally not observed.
___ is observed, however, between carbon atoms and the protons directly attached to them. If a carbon atom is attached to two protons, it can experience four different states of those protons (alpha-alpha, alpha-beta, beta-alpha, and beta-beta), and the carbon signal is split into a triplet with the area ratio 1:2:1. An additional feature of C, 13 NMR is the ability to record a spectrum without the coupling of adjacent protons. This is called spin decoupling, and produces a spectrum of singlets, each corresponding to a separate magnetically equivalent carbon atom. Compare the following two spectra, both of 1,1,2-trichloropropane. One is a typical spin-decoupled spectrum, and the other is spin-coupled
coupling
In general, ___ provides information about the carbon and hydrogen skeleton of a molecule along with some suggestion of its functional groups
NMR spectroscopy
Each peak (___) in an H-NMR spectrum represents a single proton or a group of equivalent protons. The size of each peak represents the number of equivalent protons in a group
single resonance
The number of nonequivalent ___ is determined from the number of peaks (different resonances)
nuclei
The relative number of nuclei is determined by ___ the peak
integrating
The magnetic environment of a nucleus is determined by the ___ shift
chemical
The ___ of the peak (distance downfield, or to the left) is determined by the shielding or deshielding effects of other functional groups nearby
position
The ___ of a peak is determined by the number of adjacent protons n, and the number of peaks is given by n+1 (except for 13C in the spin-decoupled mode
splitting
Ultraviolet-visible (UV-vis) spectra are obtained by passing ultraviolet (200-400 nm) or visible (400-800 nm) light through a chemical sample (usually dissolved in an inert, non-absorbing solvent) and plotting ___ versus wavelength. Higher in energy than infrared light, UV-visible light causes electronic excitations in which an electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Molecules that have more conjugation have a smaller HOMO-LUMO gap, and therefore can absorb longer wavelengths. Additionally, the higher the concentration of the compound in solution, the greater the absorbance
absorbance
Since the ___ of absorption depends on functional group of the molecule, UV-vis spectra are often used to simply identify the presence or absence of a compound by measuring the absorbance at characteristic wavelength for that molecule.
Atomic absorption and emission spectra related to the energy levels of electrons are measured in the UV-vis range, and provide information on the quantum states of atoms
wavelength
The ___ of light absorbed varies from compound to compound and depends on the functional groups of the molecules
wavelength
___ is a means of determining the molecular weight and structure of a molecule based on how it ionizes and decomposes under specific conditions. Mass spectrometry differs from other spectroscopic methods in two ways: No absorption of electromagnetic radiation is involved, and it is a destructive technique that does not allow the sample to be recovered and reused
Mass spectrometers ionize the sample in a high-speed beam of electrons. The ionized sample then enters a magnetic field that deflects the ionic fragments into a detector. The net charge and mass of each particle is determined, along with the number of each type of particle (known as the abundance). The first ion formed is the +1 cation (M+) resulting from the removal of a single electron. This unstable ion usually decomposes rapidly into more cationic fragments. A typical mass spectrum is composed of man lines, each corresponding to a fragment with a specific mass to charge ratio (m/e, also written as m/z). The spectrum itself plots mass/charge on the horizontal axis and relative abundance of the various cationic fragments on the vertical axis
mass spectrometry
The tallest peak, belonging to the most common ion, is called the ___, and is assigned the relative abundance value of 100 percent; thus the amount of any other ion is a percent of the base peak. The peak with the highest m/e ratio is almost always he molecular ion peak (parent ion peak), M+, from which the molecular weight, M, can be obtained. The charge value is usually 1; hence the m/e ratio can usually be read as the mass of the fragment
base peak
___ patterns often provide information that helps identify the compounds structure. For example, while IR spectroscopy would be of little use in distinguishing between propionaldehyde and butyraldehyde, a mass spectrum would assist in identification.
fragmentation
___ is based on the mass to charge (m/e) ratio of fragments of the molecule of interest
mass spectrometry
Each ___ in a mass spectrum corresponds to one fragment, and its height gives the relative abundance (amount) of that fragment
peak
The ___ can be identified as the highest mass/charge ratio peak in the spectrum as it is the +1 cation of the original compound
original compound
The ___ pattern can give additional information about the structure of the compound
fragmentation
