Nuclear Magnetic Resonance(NMR) Spectroscopy NMr is the most common spectroscopic tool used by organic chemists nMR differs from other spectroscopy because the different energy states (nuclear spin states)exist only in the presence of a magnetic field The nuclei of most(but not all) atoms behave as if they are spinning on an axis(nuclear spin) Because nuclei are positively charged, these spinning nuclei create small magnetic moments In the absence of a magnetic field, these magnetic moments are oriented in a random fashion For some nuclei(those having nuclear spin =+1/2), the presence of an external magnetic field (h) results in the alignment of each nucleus either wit ith (a)or against(B)the magnetic field iC and are the three most common nuclei observed using NMR spectroscopy. In this class, we will focus on H(proton)NMR Nuclear Magnetic Moments H Nuclear Magnetic Moments o magnetic field (in a magnetic field) The energy difference between the a and B states(AE)is proportional to the strength of the AE is relatively small(-10cal/mol); radio waves are used to excite(flip) nuclei from a to B E △E={954x101[rH/2r △E=h y= magnetogyric ratio cific to nucle H=magnetic field strength (at the nuck (+1/2)Nuclear Magnetic Resonance (NMR) Spectroscopy NMR is the most common spectroscopic tool used by organic chemists. • NMR differs from other spectroscopy because the different energy states (nuclear spin states) exist only in the presence of a magnetic field. • The nuclei of most (but not all) atoms behave as if they are spinning on an axis (nuclear spin). • Because nuclei are positively charged, these spinning nuclei create small magnetic moments. • In the absence of a magnetic field, these magnetic moments are oriented in a random fashion. • For some nuclei (those having nuclear spin = ±1/2), the presence of an external magnetic field (H) results in the alignment of each nucleus either with (α) or against (β) the magnetic field. • 1 H, 13C. and 19F are the three most common nuclei observed using NMR spectroscopy. In this class, we will focus on 1H (proton) NMR. α α α α α β β β β H ) Nuclear Magnetic Moments (no magnetic field) Nuclear Magnetic Moments (in a magnetic field • The energy difference between the α and β states (∆E) is proportional to the strength of the magnetic field (H) at the nucleus. • ∆E is relatively small (~10–2 cal/mol); radio waves are used to excite (flip) nuclei from α to β. β α ∆ ν ∆ [(γ·H) π]} γ ) ) β /2) α /2) ∆E E H E = h E = {9.54 x 10–11 /2 = magnetogyric ratio (specific to nucleus H = magnetic field strength (at the nucleus (–1 (+1 4